An Xlr8yourmac reader email describes some OS X Benchmarking apps from Apple:
What I thought might interest your readers is the bashpack.dmg that is installed in Developer/Applications. This disk image contains a suite of four benchmarking applications; CacheBasher for testing cache/memory bandwidth, DiskBasher for testing drive performance, GraphicsBasher for testing Quickdraw performance and Skidmarks GT for basic processor performance.
The application is available from Apple - use at your own risk.
Of note, Apple had reposted a final version of their Taking Apart Your iMac service document... but it has since been pulled off their site. Fortunately, a quick Google search reveals other sources (albeit Draft versions) of the same... for those who are intent on opening your iMac.