well that all depends on what motor we are working with... if you have a 4 cylinder, your options are pretty much slim... 6 cyl, you have a few... glasspack or even deleting the muffler could do, or a high flow muffler with no cat, I have that, and it works well.... and if you have a v8, then something's wrong if you dont have a good sound already[addsig]
I use no cat and a holley afterburner (for 39$ at checkers) its a free flow muffler and its not too loud and not to anoing, and the performance is great..[addsig]