Loop
Loop is one of the most popular free habit tracking apps, with its Android app clocking millions of downloads.
Loop is different from other apps in that it is open source. Its Github repository is available for anyone to fork and modify to their satisfaction, thereby allowing people with technical chops to make the app their own.
But Loop works great out of the box. It's got an extremely simple habit tracking interface where you can view all your habits at a glance, see how you performed in the last few days, and easily mark a habit as done, missed or skipped using taps and long-press actions.
The app also has cool analytics that lets you visualize your progress over time. What's especially great is the calendar view, which shows you at a glance which days of the month you performed or missed your habit.
All the data in the app can be easily exported as a CSV or a SQLite database file. You own your data completely.

Loop is a completely free app. It is also available as an open-source repository on Github.
Loop is available for Android.
This app is not available for iOS, Web, MacOS or Windows desktop PCs.
Habitbull
HabitBull's app has a simple but powerful UI where you can see your whole month at a glance. With this UI, it's really easy to see on which day you broke a habit.
Further, their app has graphs and charts showing you visual data on how you performed with your habits, and what your success % is as compared to your desired goals. It's even better that you can export all the data in CSV format and analyse it by yourself all you want.
One really cool feature is the ability to decide how often and how many times you need to perform a habit. It could be that you need to exercise 2 times a week, which HabitBull allows you to track easily.

Habitbull is available as a free app. You can unlock premium features starting at $4.99 per month, or pay $19.99 for a yearly subscription.
Everyday
Everyday is hands-down one of the most delightfully simple habit tracker apps out there. All the action happens on its simple visual interface, where you can see your habits stacked vertically on the left, and a timeline that progresses towards the right on top to the current day.
The core concept of the app is to perform habits daily, and to not break the chain or streak. If you perform a habit, you mark it as done. On the right, you see how long your current streak, as well as the previous high score of your streak and a total count of times you performed a habit in its lifetime.
The app is designed and built for every conceivable platform. You can use it on your Android device, iPhone, Apple Watch, iPad, Mac or on the Web.

Everyday is available for free and supported by in-app purchases that cost between $4.99 for the monthly plan to $29.99 for yearly access.
Everyday is available for iOS, Apple Watch, MacOS, Android and Web.
This app is not available for Windows desktop PCs.
Productive
Productive is a habit tracker with extensive functionality to track habits, view your weekly or monthly progress, and keep an eye on your streak for various habits.
What's great about Productive is that the app has curated a set of great habits around morning routine, sleeping better, getting stuff done etc. which makes it easy for someone new to setting habits to get started.
Another really great feature is Siri shortcuts in the iOS app, allowing you to check off habits using just your voice. The same feature isn't available for Google Assistant.

Productive is available as a free app and is supported by in-app purchases. You can upgrade for $3.99 or get a 1 year subscription for $23.99.
Habitify
Habitify keeps things simple with their straightforward habit tracker that operates on a. setting up your habits, b. getting cues that remind you to perform a habit, and c. visualizing your progress with various habits over time.
The app's features are thoughtfully constructed, taking into account real-world usage of various habits. For example, you can create a habit for meditation and also track the time you meditate in each setting, which is the behaviour of a lot of people building a meditation habit.
Another useful feature is the ability to group habits. Sort of like folders, but for a set of habits that belong to a broader goal. The app also gives you the ability to add notes, which serve as a record of your thoughts as you progress through your habits journey.
Habitify also hands-down has one of the nicest looking landing pages.

Habitify is available as a free app and is supported by in-app purchases that start at $4.99 for a monthly membership all the way to $64.99 for a lifetime membership.
Habitify is available for iOS, Apple Watch, MacOS, Android and Web.
This app is not available for Windows desktop PCs.
Coach.me
Coach.me is a holistic habit and goals tracking app that doesn't stop just there. The free app is packed with features, but the starter templates deserve a special mention. These templates have structured advice, exercises and instructions to follow, prepared by coaches, which makes the journey for someone new to this easier.
Perhaps the most unique feature about the Coach.me app is the in-built community. Not only can you ask questions about problems you're facing or feeling stuck at, but you can also share your progress and milestones with friends or community members and get high-fives from them, which serve as motivation and push you to keep going.
One additional aspect of the app is it gives you access to paid personal coaches, which might be just what you need in terms of personalized help, advice and extra accountability if things are a bit harder for you in the beginning.

Coach.me is a completely free app. You can available personalized coaching separately for a fee but you don't need to pay for the app itself.
Momentum
Momentum is a beautiful and simple habit tracker app built exclusively for Apple devices.
The core principle behind the app's design is based on Jerry Seinfield's productivity secret, which is - Don't break the chain. Each day you lengthen the chain, you make it more likely for you to stick to your habit and therefore less likely to stray from the path.
With an extremely simple and minimal UI, you can you glance all your habits in a second. Thanks to deep iOS and Watch OS embeds, you can check off a habit with one tap from the home screen or notifications shade.

Momentum is available as a free download and is supported by in-app purchases, which start at $1.99 and go all the way to $14.99.
Momentum is available for iOS, Apple Watch and MacOS.
This app is not available for Android, Web or Windows desktop PCs.
stickK
stickK is unique - instead of setting and tracking goals in an isolated way, the app has created a system of binding agreements.
As a stickK user, you sign a Commitment Contract stating that you will follow through on your newly set goals and targets by putting money on the line.
There's a psychology of loss aversion in place to not lose the money you put in, but more importantly this can build accountability to yourself that you've physically committed to your new goal, and there's actual stakes to it beyond a purely mental stake.
stickK users have put $51 million dollars on the line and collectively created 533,000 commitments. The website also says its users managed to avoid smoking 42 million cigarettes. There's power in those numbers, so perhaps it's worth giving it a try.

stickK is a free to download app, but to participate in the accountability system you have pay for the stakes involved.
Habitica
If you're a fan of role playing games, how do you feel about turning your life into one?
With a unique approach to tracking habits that is heavily inspired by RPG games, Habitica's creators have definitely flexed their creativity while building this habit tracker app.
With Habitica, the actions and tasks you perform help you gain experience points and level up, just like in a game. You can use the app with friends to complete quests or missions.
Habitica is the app embodiment of turning your life into a video game. And honestly, that might turn out to be just what you need to have fun and stay motivated while you level yourself up, IRL.

Habitica is available as a free app and is supported by in-app purchases. You can buy gems starting $0.99 all the way to a 12 month subscription for $47.99.
HabitHub
HabitHub is based on Seinfield's productivity method - Don't break the chain.
The design of the app makes it really easy to get an overview of all your tracked habits and your progress with each of those habits for the current week. Tap into any habit to see your performance for the entire month.
HabitHub's reminders work really great, taking advantage of Android built-in action buttons to a notification. Thanks to that, you can simply swipe down on your notifications shade and mark a habit as done, skip, or fail in one tap.
Perhaps the best part about the app is the ability to add home screen widgets, which let you review your current habits performance at a glance.

HabitHub is available for free and supported by in-app purchases that cost between $0.99 to $5.99.
HabitHub is available for Android.
This app is not available for Android, Web, MacOS or Windows desktop PCs.
Habitshare
At first glance, you might notice that Habitshare sports an aging UI. However, don't let that take away focus from the app's core value proposition.
Habitshare is trying to make habit tracking social. The app lets you do that by sharing your habits with friends, who can then see your progress with your habits and stay updated.
The core assumption of the is that by sharing your goals and progress with habits with your friends, you increase accountability and therefore increase your chances of success.

Habitshare is a completely free app.