The most popular current offering would be the SmoothStepper. www.warp9td.com
I've never used one, but here's what I've read.
SmoothStepper offers these features vs parallel port.
Faster pulses, up to 4Mhz, vs a max of 100khz on the parallel port, if your lucky.
Smoother pulse train, which may give slightly more speed, and smoother running motors.
USB, so you don't need a parallel port.
Gives you the equivalent of two parallel ports.
If you have a PC that outputs a good, fast pulse stream through the parallel port, then maybe the SmoothStepper doesn't offer much.
Keep in mind that the Smoothstepper is really still a beta product. Some features do not work correctly, or not the same way they do with the parallel port. It might be a good idea to join their support forum and see how it's progressing. There are a lot of users using it, though, and it shows great promise.