Ripcord is a desktop chat client for group-centric services like Slack and Discord. It provides a traditional compact desktop interface designed for power users. It's not built on top of web browser technology: it responds quickly to input, sips gently from computer resources, and gets out of your way. It does voice chat, too.
Number one feature: "Not made from a web browser."
An application that lets any laptop act as a DJ console. With it, you can mix audio files in many formats.
An open source decompilation application that helps the user reverse engineering applications. Retargetable to different CPUs, so you can (say) reverse a SPARC binary on a PowerPC machine. Modular.