A fork of the last GNOME v2.x release, before GNOME 3 went and screwed everything up. It's being maintained and updated, and suggested as a usable replacement for the bloated, lovecraftian beast that GNOME 3 was out of the gate.
Classic Shell is an open source application for Windows 8 that gives you a sane and usable desktop by replacing the Start Menu. It's skinnable, too, so you can make it look like Windows 7, Windows XP, or other versions of Windows.
The SwinSID is a pin-compatible and software-compatible replacement for the Commodore SID chip based upon an Atmega microcontroller. You can drop this chip right into a socket on the motherboard of a damaged Commodore computer, or you can probably drop it into a hardware synth that uses SID chips and hit the ground running. The firmware image and schematics are available for download for free on the website, so you can roll as many of your own as you want.
The μg Project is a collection of free and open development APIs that are fully compatible with Google's GAPPS APIs but use different back end services. For example, rather than Google Maps there is a library which uses Open Streetmaps instead, but code written to use the Google Maps API won't know the difference.