As others have said, GMATPrep is as close as you can get to the real thing. Some of the practice companies have tried to get the web interface they use as close as possible to the real thing, but GMATPrep is still the best.
That being said, don't *waste* your GMATPrep practice test just to get a feel for the CAT. You get two, and those two are probably the most accurate indicator of your performance. When you reset GMATPrep, you will likely see repeat questions and thus arbitrarily inflate your score.
MGMAT's test is pretty close as well, and you get one free practice test with that group.