One that received a lot of attention, and provides some answers to common questions is published at http://www.jailbreakme.com/. Before you visit the site with your iPhone or iPod Touch, read on.
The features list is as follows:
1. Jailbreaks iPhone/iPod Touch on 1.1.1
2. Patches Springboard to load third party apps
3. Activates non-AT&T iPhones automatically, while leaving already activated phones alone
4. Fixes YouTube on non-AT&T iPhones automatically, while leaving already activated phones alone
5. Installs Installer.app v3.0b5 on the iPhone/iPod Touch
6. Fixes Apple's TIFF bug, making your device MORE secure than it was without AppSnapp!
The site uses the known TIFF exploit which allows individuals to execute arbitrary code on the iPhone or iPod Touch by simply loading a TIFF image with Safari. Normally this is considered a "bad thing" in that virues, or other comprimising code can be deployed to unsuspecting users.
In this case, the exploit offers users a simple way to jailbreak and install 3rd party apps. It even claims to fix the TIFF exploit it used to hack your iPhone/iPod.
Obviously we need to be cautious about these exploits as it's possible that someone with more sinister motives could deploy a similar site to unsuspecting users. This particular url appears to have been tested by a number of sites, but proceed with caution.