For a list of important Cydia apps and more screenshots, go here. Visit this page if you have an iPhone 4.
There is not much to write about this time around (no PC or Mac limitation)! Jailbreaking is perfectly legal btw!
All you have to do is :
– Make sure your iPhone is activated. It is not possible without a valid iPhone SIM. If you don’t have one, we must wait until another tool is released.
– Make sure you sync your iPhone to iTunes. This is important, just in case you know!
– …

CAUTION : iOS 4 jailbreak is way complicated unlike other jailbreaks. Please proceed with caution.
Requirements :

Jailbroken iPhone 3GS with older bootrom.
You haven’t used Spirit jailbreak before (for untethered jailbreak).
You have Windows (PC). For Mac, try Pwnage instead.
Please read the above requirements again.

More info about these custom firmware files :

[UPDATE] : You need to download files from either step 1a or 1b depending on your baseband preference. Both are unlockable, so I would go with step 1b which is the latest baseband which supposedly does better with the battery. step 1b …

UPDATE : The following guide holds good only for iPhone 3GS 3.0 firmware. For 3.0.1 use redsn0w guide instead!
Geohot has done it again! He gave us the first jailbreak for iPhone 3GS and now, we have a much more efficient iPhone 3GS unlock – purplesn0w. Seriously, all the sn0w versions are confusing but well, that is their game!
For now, the iPhone 3GS should be activated using an official carrier before purplesn0w is installed on your iPhone 3GS. (Hacktivation for other carriers to follow soon)
Why is this unlock (purplesn0w) more reliable …