mwave offers the refurbished Seagate Barracuda 7200.9 500GB Serial ATA 3Gb/s 3.5" Internal Hard Drive, model no. ST3500641AS, for $49.99 with free shipping ($0.10/GB). That's $25 under the lowest total price we could find for a new one. (It's also the least expensive 500GB internal hard drive we've seen this year.) It features a 16MB cache and runs at 7200 rpm.

Note: A 60-day warranty applies, although it's unclear who supports it.