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 & iPod Touch 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.
You'll find Cydia™ right there on your iPhone after you jailbreak your iPhone. Using it is easy - browse the categories, check what's new, or peruse the featured section to see the current popular apps. There's hundreds (getting into the thousands) of themes to choose from. Cydia™ is easily searchable via the iPhone - but if you're wanting to browse what's available from your browser.
OR copy deb file into DIR var\root\mobile\media\Cydia\AutoInstall, and reboot iPhone/iPod Touch
You can download Cydia (deb, v1.1.23) here