For OS 10.5 up, there's the CLI "memtest" and the GUI front end for it "Rember".

http://download.cnet.com/Rember/3000-2094_4-52452.html?tag=mncol;1

Here's a version for earlier OS X (shareware $1.39):

http://download.cnet.com/Memtest/3000-2094_4-32926.html?tag=mncol;5

Nearly all benchmark suites have a disk drive test, so pick your favorite.