Drive Testing and Diagnostics?

Can anyone reccommend a good (FREE) all-around drive testing and diagnostics utility(ies) for USB Hard-drives? I want to benchmark and compare my 4 USB Drives and see which is best. I have four. (eek, I know). I would just like to compare their performance and their specs. I built 2 of them (kits) and bought 2 pre-built (WD and Maxtor). Thanks.

Jacoby