Weblog Lifehacker has an excellent rundown of the 20 best iPod utilities. These programs will make your iPod run better, faster, harder, stronger as well as achieve feats no normal iPod was designed to achieve.

Here are seven, in no particular order:

• How to jailbreak your iPhone
• Wirelessly Stream Music from Your iPod touch or iPhone to Any iTunes Library
• Install Linux on Your iPod with iPod Linux
• Put Wikipedia on Your iPod with Encyclopodia
• Upgrade Your iPod Video to an iPod classic
• Backup Your Home Folder to Your iPod with iPodBackup
• Convert YouTube Videos for Your iPod with Zamzar

Obviously, there are plenty more items, and plenty of description in the original article, here:
The 20 Best iPod Utilities