I've been using a Xitel INport for my turntable for years and love it. I recently bought a second one on eBay to replace the phono to 3.5" Y cable I've been using for my cassette deck and am very pleased with that decision as well. INport retails for about $80, same as the unit Don references, and the retail unit comes with a 30' (yes thirty foot!) shielded stereo phono cable. Although they're very popular, you can acquire a used unit at much less on eBay if you're patient, especially if you don't need the cable and just want to buy the adapter.
Don't know about other brands of USB converter, but the Xitel unit has very good signal to noise and the analog to digital conversion before the signal hits the PC means you don't have any issues with ground loop hum, cross talk from other sources etc.
Apple Mac users seem to swear by Griffin Technology's iMic product. You don't care that it doesn't come with PC software since you already have the best
The Griffin product only has a 3.5" input plug, but they supply a stereo phono to 3.5" adapter.