I have a jailbroken iPhone 3G.
These are my top apps (I didn't install Rock. I only have Cydia):
Backgrounder and Multiflow (allows me to multitask with a decent UI for all the open apps.. but only up to 3 or 4 apps as the iPhone 3G's memory is limited.)
SBSettings (kind of like a CCleaner for the iPhone which dumps out trash. pretty neat.)
Cycorder (allows me to capture a good quality video for my phone)
Installous (Thousands of FREE Apps! It's like a substitute for the app store.)
Winterboard (for themes and customizations)
and many many more. my iPhone 3G has become so much better with jailbreak.