I keep a special folder of Mac OS X diagnostic and repair applications on my Mac's hard drive in case of emergency and there are a couple tools that every self-respecting mobile Mac user should have in their arsenal. This is a short list of some of my essential selections:
The conventional wisdom is to run Disk Utility's a Repair Disk Permissions operation regularly and before and after a Mac OS software update but I'm increasingly starting to agree with John Gruber that Repair Permissions Is Voodoo.
Instead, I keep a complete clone of my hard drive on an external Firewire hard drive and keep it at home for safe keeping. I make the initial clone to the external using Mike Bombich's excellent Carbon Copy Cloner, then I synchronize the newer files weekly with Qdea's Synchronize Pro X (US$100).
If I suspect that my machine's acting a little wonky I run Jonathan Nathan's Preferential Treatment which checks all my preference files (both user and system preferences) for corruption by using the "plutil" command line tool.
If I have a permission issue (which happens often if you have multiple Mac OS X accounts on one Mac and share iTunes and iPhoto libraries between them, for example) Gideon Softworks' FileXaminer (US$10) can usually fix them without a trip to the Terminal.
Tiger Cache Cleaner (US$9) provides easy access to dozens of OS X maintenance and utility chores, including: repair permissions, run maintenance scripts, rebuilding prebindings, cleaning system logs and Spotlight meta data, eliminating duplicate or orphaned Login Items. TCC can be configured to run on an automated schedule.
Lastly, if you really get into a bind and lose a hard drive that's not backed up, my go-to recovery utility is ProSoft's Data Rescue II (US$99), especially now that they released a Universal Binary version and a bootable CD option that works on Intel Macs.
What's in your Mac tool belt?