I was suprised to find that I could actually play an online game of Snoopy Flying Ace (XBLA) connecting to XBL through my iPhone 4's 3G connection (with MyWi)! I don't know if others saw any issues in the game, but for me it was smooth as butter. I even got some kills (Snoopy Flying Ace is an awesome game, by the way.) I was mainly just wanting to try it out, but when it worked great I saw the game to completion. Probably won't do it again, especially since our home internet should be back up by Monday evening at the latest.
As far as what I have installed via Jailbreak on my iPhone 4:
My3G - paid - like 3G Unrestrictor, and has worked well for me but I haven't tried Facetime with it. I think 3G Unrestrictor is free, so that's something to consider. I like how My3G works, but I haven't tried 3G Unrestrictor so I can't compare.
AppBackup - free - lets you backup and restore the save files from AppStore apps. These saves are on device, so it's not really so much for backing up to safeguard against a device failure, but for things like backing up before deleting an app, which deletes its data, and then being able to restore that data later on when you reinstall it. So far I've only backed up, not restored, but so far so good. I'm going test out restoring soon. Could also be useful for saving a state and then being able to revert back to it in a game. Not sure.
Texas Poker - free - note that this is also available via the AppStore, for free there too
FinderMobile - free - lets you browse the files on your iPhone. Not really that useful as far as I can tell - it doesn't appear that you can do much with the files. You can open some kinds of files, I believe, but you can move or copy things or anything of that sort. Destined for deletion at some point....
gpSPhone - free - GBA emulator. I haven't really tried it other then to launch it, but reportedly it works. You have to copy your roms onto the phone via other means.
UnlimTones - free - direct download of ringtones. You can download them and use them right away. I downloaded a Super Mario Bros. and a Zelda ringtone. Probably somewhat questionable, but I've got the games they are from. This just saves me the trouble of having to record the sounds myself.
RockApp - free - another jailbreak software store. I like it more then Cydia - it's faster and has a better GUI, except that not all paid software is available for purchase from within it. MiWi and Mi3G require RockApp.
ScreenDimmer - paid - provides customizable display dimming (strangely missing from the iPhone/iPod touch) as well as options such as turning off screen locking when the iPhone is powered. Simple, but I love it, and it's cheap.
AutoCorrection SBSettings add-on - free - gives a toggle for autocorrection
Autolock SBSettings add-on - free - a bit redundant given that I have ScreenDimmer, but might come in handy at some point
Already covered by Malik:
MyWi - paid, but so worth it
SBSettings - free
Activator - free
CyDelete - free
OpenSSH - free
I also tried BTstack GPS, but it seemed to cause instability. It is suppose to add support for external bluetooth GPS receivers, with support for any App on the iPhone. Hopefully the instability can be fixed as I've ordered a bluetooth GPS unit that should give me better GPS reception. In any case I want to try it out and see if it makes it better or not. I especially want it for GeoCaching, want to use it with the official Geocaching app. Also navigation, of course.