Well, I'm not Pete, but I've got some experience in this area being a Jeep owner (we like to make modifications
). The laws are different in every state, but...
-There usually is a law for headlight height
-There are often laws for changing the exhaust, but most comercially available mods are street legal, it's when you start with the custom fabrication that things start to get illegal
-There are often laws against running open headers and other mods that bypass exhaust systems all together
-I believe nitrous is always illegal for street use, which a lot of street racers use
-There are definite regualtions about how low or high a vehicle can be, but these are normally done by headlight as stated earlier
-There are also all kind of laws that they would have a really hard time enforcing like open vs locked diffs, octane of fuel, bead lock wheels, etc.
For the most part many of these laws are not enforced, at least not in my experience. From what I understand though, they are pretty tough over in California (all those environmentalists I guess).
Hope this answers some of your questions and Pete can feel free to correct me on anything I've said, everything I know is just from personal (and friends') experiences.