If you've upgraded recently to firmware 1.1.3 on your iPhone or iPod touch, you'll probably have first downgraded to firmware 1.1.1 and used the popular jailbreakme.com site. Well, it looks like jailbreakme has undergone some massive updates to make this process easier and more optimized. It now automatically oktopreps your 1.1.1 installation, installs the latest Installer.app version (including community sources so you get the new modmyifone source), and cleans up after itself more thoroughly (.svn and youtube.zip files anyone?).
Once you've done the 1.1.1 downgrade bit, be sure to check out the 1.1.2 upgrade guide as well. You'll quickly be on your way to 1.1.3 happiness. Doesn't seem to be working? Make sure you've connect your iPod or iPhone to your computer via the USB cable. There's a handy FAQ available that answers other common questions.