On top of all of the suggestions here, browse CD-Media. It will take you several hours, but you will find
drive trains you're looking for. Once you find a
good pic, hit the little heart icon so it saves it to your favorites for future reference. More importantly though, you'll have a better idea of what questions to ask.
Another alternative is
linkage drive.. It has the benefits of strafing & diagonal movements while also being simpler to implement than most common crab drive trains. This year (2009) my team did that drive train with 4 independent pneumatic pistons for the linkage actuation, and I must say that it was the best decision we've ever made post-kickoff. It was easy to prototype in 2 weeks, and has been the most reliable drive train we've created to date (though I wonder if that has to do with the fact that we have no chains...). I does have its cons, such as the fact that we think our current specific design would explode with high traction wheels on carpet, but there are challenges to overcome in any design if you think about it.