I won't mention any jailbreak apps specifically, since they've already been mentioned.
The key to the jailbreak community is that this is where most of the key features in iOS come from. When there wasn't copy & paste, jailbreaking provided us with this feature. Notification center, text shortcuts, multi-tasking, etc. So while Apple may have recently implemented notification center, there will be more to come from jailbreak developers. A lot of creativity comes from the jailbreak community.
MyWi is the most important for me but SBSettings is one of most used because the phone freezes so much that I have to use it to force off apps. Maybe if I wasn't JB'd it wouldn't freeze so much to begin with but I definitely still need MyWi. Also FreeSync is super useful and iCallAnnounce I use a lot specially for incoming texts so I don't have to get up and go see who it is texting me. ManualCorrect hopefully they included in ios5.
I suppose if I wasn't JB'd I wouldn't need PkgBackup or RecoveryGuard.
The only real thing I absolutely NEED - and which requires a jailbreak, is the ability to define a "night profile" that will cause the phone to turn off all sounds except ringtone and text alert at ~10:00PM, and revert to the "normal profile" with email alerts and everything else enabled at 08:00AM. Without this, I ALWAYS forget to turn off email alerts when I go to bed, and the first email I get wakes me up (and pisses me off).
I use the text-alert as my work pager, so this has to stay nearby 24X7 and must always awaken me. So I can't just silence the phone when I go to bed.
I usually end up just never using email alert, because remembering to turn it off is a pain.
An extension of the "profile" idea using geo-fence would be to change to a "professional profile" whenever I am near the office. Thus when I'm at work my email alerts would be subdued beeps rather than the doctor from SeaLab 2021's "Predator" episode saying "Ohhhh Goodness!".
I used to be jailbroken, but I haven't been in a few weeks now. I really like the ability to arrange my icons any way I liked without them snapping back into place. Folders within folders was nice to for a clean background. I will probably jail break again once an untethered one comes out for ios 5, but for right now it hasn't been that big of a deal being regular again. I have less technical problems with my phone now. So i guess that's a plus.