Linux Live: If you want to put your computer through the paces and get some feedback and benchmarks, you'll be hard pressed to find a tool more packed with tests than the Phoronix Test Suite Live CD.
PST Desktop Live is an Ubuntu Live CD with the Phoronix benchmarking suite preconfigured and installed. Reboot your machine with the CD in your drive — or on a USB drive — and you'll boot into the test suite with access to 52 tests and 10 profiles. The profiles are set up to help you stress test and benchmark based on what you'll be using the machine for.
The desktop interface and suite is set up so that even users unfamiliar with Linux can jump right in and start testing without having to do any special modifying or downloading of additional files. You can save your benchmark results or share the results online with the Phoronix Global benchmark sharing system.
Have a favourite benchmarking tool or Live CD packed with goodies? Let's hear about it in the comments.