ZMA is a good supplement, and can be purchased widely for cheap. It raises testosterone naturally with minerals.
Tribulus seems to be more associated with libido than testosterone. The verdict is still out on this one.
I've heard a couple of positive remarks about 6oxo, but honestly I don't know enough about it to give an opinion. I know it's an anti-estrogen. It's advertised as an aromatase inhibitor. Aromatase catalyzes the transformation of aromatizable androgens into estrogens, so less aromatase equals less estrogen. Whether or not it does what it claims I don't know.
Here is a purported 'study', but of course you need to take it with a grain of salt:
http://www.bodybuilding.com/store/ergo/6oxoresearch.htm
Other than the 6oxo unknown, the question is whether or not you'd be better off buying some ZMA on your own, since it's cheap. Also, how much ZMA is in this product?