Disk Utility is a great maintenance and troubleshooting tool for those times when your Mac starts acting up. Repairing permissions can sometimes be handy (even though its true validity is in question as of late), but using the Verify and Repair Disk operations is often a more useful tactic. However, until Mac OS X 10.4.3, you had use Disk Utility to verify or repair your Mac's drive from some sort of an external source. You either had to plug into another Mac in Target Disk mode, or reboot and use the install disks that came with your Mac - not the most friendly process to your workflow. But now, With 10.4.3 and later, you can start up Disk Utility right from within Mac OS X and chose the verify option to make sure system level elements like your file catalog are all in order. One warning though: it is completely normal for your Mac to temporarily freeze or 'seize up' at the beginning of running this operation (you should get the infamous beachball), as this Apple support document explains. After a little while, depending on your Mac's speed and all that jazz, it will come out of its daze and you can continue working/chatting/surfing while it finishes the disk verification process.
This is a really handy tool, perhaps even more valuable than checking permissions, and should be a welcome addition to any Mac maintenance arsenal.