AppCleaner
AppCleaner helps you to find all files related to the app so that you can remove all of them at once.

Bitwarden
Bitwarden is free, useful, and secure open source password manager. It is compatible with Mac, Windows, Linux, Android, and iOS. You can also use it as a browser extension.
Clipy
Clipy is a free and open source clipboard extension app. You can use it from a menu bar icon or with a shortcut key cmd + shift + v. You can also use it as a snippet manager.
CotEditor
CotEditor is lightweight open source text editor with many useful features. You can use it for coding.
coconutBattery
coconutBattery.

draw.io
draw.io is a free and open source diagram software for making flowchart, network diagrams, and so on. The UI is very simple and easy to use. You can use it from a browser or desktop app. You can put mathematical equations in the diagram with .

Hand Mirror
Hand Mirror is a free app for checking your face with your Mac camera. You can open it from a menu bar icon or with a shortcut key.
ImageOptim
ImageOptim is a free and open source app for compressing images. You can compress images without losing quality. You can also use it from the command line. For example, you can compress all images in the current directory with the following command:
$ open -a ImageOptim .Mathpix Snipping Tool
Mathpix Snipping Tool is a free app for converting images to LaTeX. You can convert images to LaTeX by taking a screenshot: cmd + ctrl + M.
You can also make a latex equation image from a menu bar.

MonitorControl
MonitorControl is a free and open-source app for adjusting the brightness of your external monitors. You don't need to use the monitor's buttons anymore! You can adjust the brightness with shortcut keys.

Rectangle
Rectangle is a free app for resizing application windows. You can use Rectangle from a menu bar icon, with shortcut keys, or by dragging the window to the edge of the screen.

Shottr
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Shottris a free app for taking screenshots. You can take a screenshot of a whole screen:cmd + shift + 1, or a selected area:cmd + shift + 2, an active window:cmd + shift + 8, or a whole page by scrolling:cmd + shift + 9. - You can edit the screenshot immediately after taking it. You can blur sensitive information, erase unwanted parts, add arrows, highlight texts, add text boxes, and add rectangles.
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The best feature of
Shottris that you can use multi-language OCR to convert the screenshot to text:cmd + option + ctrl + o.
Syntax Highlight
Syntax Highlight is a free and open source app for highlighting the syntax of source code in Quick Look (press space key on a file).
Unsplash Wallpapers
Unsplash Wallpapers is a free app for setting beautiful wallpapers. You can set a new wallpaper from a menu bar icon.