If you buy a pre-made external drive, any major mfr will be good -- Maxtor, Western Digital, etc. The differences are so subtle in consumer-grade products, it's splitting hairs to compare them.
If you make your own by buying a hard drive and a separate enclosure, be careful. #1, make sure the enclosure has a fan if the drive will be on for more than several hours at a time. #2, make sure the hard drive is designed for back-up. Many drives are not built to handle sudden, mass bursts of data on a regular basis as most hard drives read and write small amounts.
Either way you go, I'd definitely recommend getting some type of network connectivity if you happen to be running a home network. Also, go bigger than 100gb -- that's chump change these days and you can get a 250gb for so little.
Alternatively, you could always switch to Mac and subscribe to iDisk.