Here are some projects that I have worked on over the past few years. All of them are free and the code is open-source under various licenses.
CacheView is a simple web app that finds cached (backed up) copies of a specific web page from the Wayback Machine, Google's web cache, and Google's Accelerated Mobile Pages (AMP) cache.
Peek allows you to preview dozens of types of documents, without downloading them. Just hover over a file's link, and Peek will display an interactive preview instantly. The extension supports PDFs, Office documents, videos, audio, Google Docs links, and more.
This extension integrates Google Tasks with your web browser. A shortcut to view and manage your tasks is added to the toolbar, and on Firefox, Tasks becomes accessible from the sidebar.
Browser plugins are unsupported by modern browsers, but what if you come across a QuickTime video or a music page using Windows Media Player? NoPlugin allows you to play some plugin content by converting them to HTML5 players. If NoPlugin can't play a file, it can download the media file to your computer.
Wikipedia Search adds the ability to search Wikipedia straight from the address bar. You can also right-click a selected word or phrase to perform a search.
This downloads and installs Adobe Creative Cloud using Wine, allowing you to use recent Adobe software (like Photoshop CC 2018) as easily as you would on Windows.
Tootbot is a Python application that automatically mirrors Reddit posts to Twitter and Mastodon, with media attachments for images/GIFs. It can be run locally on a computer, or in the cloud for free using Heroku.
Nexus Tools is an installer for the Android debug/development command-line tools ADB (Android Device Bridge) and Fastboot for Mac OS X and Linux. It's the fastest way to get started with everything from Android development to fixing your phone. Just enter a single command, and Nexus Tools will do the rest.