The jailbreak itself lifts the app limitations apple puts in and installs Cydia - the homebrew app store. I thought that it's all free packages but it's not. The good ones aren't free, and some are damn expensive in contrast to the 1-2 dollar stuff from the app store.
There are a few I wanted.
For one, you know you can't do facetime over 3G. Well the reason for that is that they're working on making that a pay option to add to your monthly bill. The My3G app will let you facetime over 3G.
MyWifi or MyWi, can't remember, turns your iphone into an access point (tether over wifi).
I miss having my winmo device show my upcoming calendar events on the today screen. NOthing in the appstore will do that. Intelliscreen will let you configure to show stuff like your upcoming calendar events, rss feeds, new email snippets, weather, on the lock screen. But for 10 bucks? **** that. Apple will add that to a future update, I know it.
EAlert will pop up a new email notification/snippet no matter what app you're in (instead of just the notification bleep you have now).
There's more (like ssh).