there's also the option to put a silencer into your exhaust... though not usually the first choice of many (or all, I would personally imagine.)

Apexi' makes a product called a ATS (Active Tail Silencer), which is kinda cool, it's like a silencer valve that opens up when given a reason to (i.e. higher rpm exhaust output) so it's silenced down and quiet when it doesn't matter, but open and free flowing when it counts...