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.
WhatDevice is a web app that displays information about your device and browser, currently in beta. You can view the source code here.
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.
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 script makes installing Adobe Creative Cloud on Linux super easy. It downloads and installs 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.