My jailbreak killer applications:
Backgrounder - Multitasking, allows you to keep applications running as long as RAM allows. Fine on 3Gs, limited on 3G. I suspect we will see apple multitasking in version 4.0 and it will only support 3GS and the successor but that is pure speculation
Offlinemaps- Bring all your maps of foreign countries with you, googlemaps or other types and all work in the maps application
PDAnet for tethering
Unlocking
SBsettings
Rotation inhibitor (lets me stop it rotating which is good if putting it in a motorbike tank bag running a maps application as otherwise the maps keep spinning about)
Mobile terminal