Updates:
- I’ve posted a new Top 10 list for 2010! Revised Top 10 Cydia Apps for 2010
- If you’re looking for a quick iPhone jailbreaking guide, I’ve compiled a list of steps for iPhone OS 3.1.2 (and prior) jailbreaking at this post here: iPhone Restore Instructions: v3.1.2 + Jailbreak
Jailbreaking your iPhone allows you to install unauthorised apps that Apple have not yet allowed (for example unlocking or video recording).
Once you’ve jailbroken your iPhone you’ll notice a new app in your Springboard, called CYDIA. Cydia is the hackers version of the iTunes App Store. Once you open Cydia, you’ll be bombarded with the biggest list of apps EVER most of which are free. Whilst this is awesome, the sheer volume of apps makes it very hard to make educated choices on what apps to install, and it’s very easy to install totally useless apps and waste your time. So we’ve compiled a beginners guide to Cydia, 10 apps that will get you going with your jailbroken device and make your iPhone life easier and more useful in the process.
Williamo's Blog 
Pretty informative i might say so myself. I liked it and found another source of info take a look at riwoken.
Nice! My favorites iare MX tube and MyWi