IOS 4: FontSwap
IOS 5: BytaFont
After downloading the one that you need to download, respring your device. once you've done that go back into Cydia and download the font that you feel is right for you. KEEP IN MIND!! Only FontSwap fonts work with FontSwap and only BytaFont fonts work with BytaFont. After downloading your fonts, go into the application and apply them and then respring your Idevice and your done.