You can rent from most of the major chains under 25, but there's normally an extra fee (which normally isn't that bad honestly). However, most places won't let you rent an SUV or something along those lines.
If you can rent from National, you can pick any car you want on the lot which is pretty cool (except for the "executive" section). I also think Budget will let you rent SUV's. Just make sure you have an actual credit card and don't let them talk you into extra insurance if you're staying in the country, most US insurance companies will also insure you if you rent a car.