Too bad the load from the thousands of jailbreakers is bringing it to its knees right now. It should be working fine in 2-3 days.
Once it is up, these are the tweaks to download.
Monocole- run any app in full screen.
Auxo- amazing multitasking alternative. Must download.
AnyLockApp- change the slide to open camera on the lockscreen to any app you want.
Flux- changes screen warmth to match light in room. This is one of my favorites.
iFile- browse iOS filesystem. My absolute favorite jailbreak app, bar none. You'll need this to flash PRLs (once someone or I figures out how to do it).
SBSettings- access Settings app toggles from home screen or anywhere. Must download.
Winterboard- this isn't compatible with iPhone 5 yet, but it allows you to theme your device. Must download.
SwipeBack- simply swipe to the left on the screen to go back in a menu, rather then press the back button.
Accellerate- make iOS animations faster.
Activoice- Open Google Now (yes, Google Now) with a simple gesture, whether it be a shake of the device, or another gesture.
There's a lot more that I love, but those are my favorites.
EDIT: Wait one more!
BiteSMS- Alternative to the basic Messages App. Adds tons more features, including Quick Reply.