The ethernet smoothstepper is a classic. it works great and basically provides 3 parallel ports worth of i/o. should be able to use the existing bob or can upgrade to something like the pmdx-126. cnc4pc also has a variety of bobs designed to plug directly into the smoothstepper.
pmdx also has some new inexpensive usb motion controllers out, one of which would plug right into your bob. i've had a chance to play with the 422 with integrated bob, super easy to set up.
then there's the cslab csmio, vital systems hicon, and a host of others. depends on what you want and how much you want to spend. The smoothstepper and pmdx boards are under $200. a hicon integra without closed loop will run around $500, and is worth every penny. it is a much more industrial level motion controller.