ZMA doesnt seem to do much for a lot of people. I know a few people that have tried tribex-500 by biotest(tribulus and some otehr stuff) and have seen some results from it. You cant expect huge results from a lot of the natural test boosters, some OTC things work fairly well though(like 1-ad).
I've been taking tribestan for a while and notice some gains - not sure if its from my workouts or the trib but you are suppose to take it for the full 90 days - to really see any results.