Building Muscle

I have been exercising like crazy recently. After completing the intensive p90x and now 1/3 of the way through Beach Bodies Insanity I am starting to see some really positive results. The hard work is paying off and I have never felt better.

I'm not that serious about being muscular but am getting more define. However if I was more serious then it would make sense to take a good look at muscle builders and see if there was any supplements available that could help me build the body I wanted. Admittedly my vegetarian diet does lack protein so a supplement may be able to help me in this area.

What supplements do you use?