I'm eagerly anticipating the day when I can show you a 1-terabyte hard drive for under $100. (Be patient, it'll happen.)
In the meantime, eCost has the 500GB Vox V1 external USB/eSata drive for just $55. There's a rebate, yes, and shipping will run you about 10 bucks. But, lordy, that's a lotta storage for notta lotta cash.
Drives like this are ideal for making full-system backups, storing massive video libraries, and just expanding your available storage in general. CNET loved the V1's ease of setup and speedy performance, but wasn't wild about the bundled backup software. Of course, you can always switch to another utility if you prefer. (Check out Paragon Software's Drive Backup 9.0 Express, which offers full-system backup and doesn't cost a penny.)
The V1's specs include USB and eSata interfaces (plus cables for both), a 7,200-rpm drive, Windows and Mac compatibility, and a one-year warranty.
Except for having to wait on the $40 rebate (PDF), it's a pretty killer deal.