Cydia
What is Cydia?
In short, Cydia is like the App Store of jailbreak applications. Just like the App Store, you can browse Cydia for apps, tweaks, and mods. Just like the App Store, you can download and install Cydia apps effortlessly. Most apps and tweaks in Cydia are free, but it is not unusual for a jailbreak app to sell for a few dollars.