Is your Mac acting funny? If so, you can use the install disks that came with your machine to test your hardware. To use Apple's Hardware Test (AHT), insert install disk #1 and reboot your Mac while holding down the "d" key on the keyboard (Intel machines only).
The AHT software will load and ask for your language -- choose your correct language and click the arrow. From the main menu you can click the "Hardware-Tests" tab. Two tests are available: standard and extended. The standard hardware test will allow you to do a quick, 1-3 minute test of the memory and logic board, while the extended test will run an hour-long test of your system. You can get to the extended test by clicking either a button or check box (this depends on your AHT software version).
Want to see more tips and tricks like this? Check out TUAW's Mac 101 section.