Sure, the App Store is filled with a plethora of great applications but there is a growing community behind apps that will never be approved by Apple Inc.
Here's a list of apps worth jailbreaking for.
WinterBoard
WinterBoard is probably the most popular app within the Jailbreak community. Developed as a replacement to SummerBoard, Jay Freeman has been working hard to improve it. With WinterBoard, you can customize almost every single aspect of your iPhone GUI. You can change icons, wallpaper, dock, sms balloons and if you want, use a video as your wallpaper.
Cydia provides a repository of themes that are free for download so less tech-savvy folks do not have to worry about SSH.
StatusNotifier
StatusNotifier will notify you when you receive a new Email, SMS, MMS, IM, Call, Voicemail or Calendar Event. By default, these notifications will appear on your status bar but with a bit of tinkering and hacking, you can even set it to show on your "Slide to Unlock" screen. In order to do so, knowledge of HTML, CSS and Javascript is required.
MxTube
In short, MxTube allow you to search and download YouTube videos. It works as advertised! Now you can share that funny video with your friends while commuting to school!
dTunes
dTunes allow users to playback downloaded media without the need to sync in advance using iTunes. Coupled with the SafariDL plugin, you can decide to save or playback a music or video file when you encounter one in Safari. There is also a built-in browser allowing you to search for music and videos.
The latest version of dTunes will even allow users to unzip files.
MPlayer
MPlayer is a media player capable of handling formats such as mpeg4, h.264, wmv, mp3, aac, wma, real media and t-dmb. It is actually a port of the cross-platform media player for Windows, Linux and Mac OS X.
Conclusion
I hope these applications will be of great use to you guys. For more jailbroken apps, Lifehacker and ReadWriteWeb have compiled a list as well.