Here's my favorite creations that I've worked on over the past few years. All of them are free and their code is open-source under various licenses.

Browser Extensions

Wikipedia Search
Wikipedia Search is an extension for Google Chrome and Opera. It adds the ability to search Wikipedia straight from the address bar, making searches faster and easier. It supports searching in every language, searching any word by right-clicking it on a page, and more.

Peek is an extension for Google Chrome and Opera that lets you preview dozens of types of documents before you download them. Just hover over a file's link, and Peek will display an interactive preview of the file instantly. It shows previews for PDFs, Office documents, videos, audio, Google Docs links, and more. See it's GitHub page for more details and the code.

Browser plugins are unsafe and unsupported by modern browsers, but what if you still need to access plugin content online? NoPlugin allows you to play some plugin content on Google Chrome and Opera by converting them to HTML5 players. If NoPlugin can't play a file, it lets you download the file to play on your computer. See the GitHub page for more info and the source code.

Other projects

There are dozens of JavaScript libraries for making online video players (like YouTube videos) fit the page, but I didn't like any of them, so I made my own. It's incredibly lightweight, works with every major online video player, and doesn't need any other libraries.

Cobalt OS
Cobalt is a free and open-source operating system, based on FreeDOS, aimed at making DOS as easy to use as possible. It has features like an optional desktop, support for CD drives and long file names without messing with drivers, and an incredibly simple installer.

Nexus Tools
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. It's even been featured on XDA Forums!