Recommended (off the top of my head):
AptBackup
This compiles the list of apps/tweaks/themes you've installed via Cydia. It doesn't actually back up any apps themselves or any associated data, but it does create a little file that, when you sync your phone (jailbroken or not) gets saved in your iTunes backup file. If you ever lose the jailbreak or restore, AptBackup will reinstall all of your packages/jailbreak apps in one step. (
more info)
BiteSMS
Widely discussed. I will say that this app is worth jailbreaking for, all by itself. (
more info)
Call Bar
Stops incoming calls from interrupting you in whatever you're doing by notifying you through the notification center at the top of the screen. (
more info)
Five Icon Dock
Just what the name implies, it squeezes five app icons into the Springboard dock instead of four. (
more info)
IntelliscreenX
If the iOS notification center is a little league diamond, this turns it into a major league stadium. It's a feature-rich, paged expansion of notification center. (
more info)
Lockscreen Dim Delay
Allows you to specify how long the backlight stays on during lockscreen notifications, from 1 second up to 30 minutes ...for those who hate their batteries. I like to have 15-20 seconds set for mine. (
more info)
My3G
Another favorite, My3G unshackles your apps from the restrictions imposed by being on a carrier network. That is, for example, YouTube dumbs down video quality when you're not on WiFi, and the App Store will not allow you to download apps over a certain size until you connect to WiFi. Most importantly, it allows Facetime use over the network. Essentially, it tricks your apps into thinking they're on WiFi when they're not. Useful. (
more info)
WeatherIcon
I use this to put the current outdoor temperature and weather conditions in little icons on the status bar. (
more info)
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There are hundreds and thousands of tweaks, apps, themes, etc available, but these are always good starters with which to baptize yourself into the world of jailbreaking. All of these are available through the installer known as "Cydia", an app you'll find when you first jailbreak.