Cydia™ is Debian APT on the iPhone. Think AppStore, but with packages you could never get direct through Apple.

Created by Jay Freeman (saurik), Cydia™ is a feature-rich AppStore competitor. Full themes to completely make over your iPhone's look are available through WinterBoard. Video recording is possible using Cycorder or Video Recorder for 3G. You can share your iPhone's 3G connection with your laptop using PdaNet, or get free music, videos, and torrents using dTunes. Take complete control of your iPhone with SBSettings.