Well, for pre-built ones, Seagate or Western Digital are about all I'd buy.
I wouldn't buy a pre-built one though. I have a AMS Venus D3 USB Enclosure case. They have versions for IDE-->USB, IDE-->USB+IEEE1394 (FireWire), and SATA-->USB+eSATA. They're available on NewEgg (a great, reliable, cheap online vendor), and they work great and its mind-numbingly simple to assemble the drive into it!
Here's one that'll work with a regular Parallel ATA IDE drive (the regular, "old" style of hard drives):
http://www.newegg.com/Product/Product.asp?Item=N82E16817145656And here's the one that'll work with a "new style" SATA drive (either SATA1/150 or SATA2/3.0Gbps):
http://www.newegg.com/Product/Product.asp?Item=N82E16817145660That way you can buy whatever internal drive you want (again, I like Seagate and Western Digital) and: a) save a little money, and b) upgrade your capacity later if you need to. They also have the handy benefit of allowing you to temporarily swap in another drive. I use this all the time with mine. I have a bunch of internal disks that I use for different purposes. Some of them I don't need very often (backup and whatnot) and I store them in the closet and only pop them into the case when I need something. Also, if someone brings me a computer that won't boot or something, I can use the case to grab info off the drive with a working computer. Lots of useful stuff!
Here are some current Parallel ATA IDE drives:
PATA100 (IDE) Western DigitalPATA100 (IDE) SeagateHere are some current SATA drives:
SATA1 (150) Western DigitalSATA1 (150) SeagateSATA2 (3.0) Western DigitalSATA2 (3.0) SeagateNOTE: The speed of the drives (the SATA 150 vs. 3.0, and really the PATA vs. SATA) makes no difference if you'll only be using it inside the USB enclosure, as my tests mentioned above show. Any internal drive is going to be fast enough that it will be much faster than the USB port can handle traffic. The biggest thing is that companies aren't really releasing big PATA drives anymore, so if you want to be able to upgrade to a bigger drive in the future, you'll probably want to go with the SATA case. For example, Seagate has a 750 GB drive now, but it's only available in SATA. Western Digital doesn't have a 750 yet, but it and 1TB drives are coming soon, but will likely be SATA only. For drives in the enclosure only, SATA1 vs 2 makes absolutely no difference. Just pick whatever one's cheaper (and make sure to check SATA2 drives because sometimes they're cheaper than SATA1 oddly).
EDITED to add internal drives and more info.