In the US it seems you can have just about anything you want that fits in the space, presumably with some rules about not being brazenly obscene or something.
In the UK it's more interesting because number plates follow a pattern so you can't just have seven letters. There are also rules about how the letters and numbers are to be spaced although many personal plates ignore them. From what I can tell the police don't tend to enforce that rule because the purpose of a plate is to identify the vehicle and if it's spaced to make it more memorable then it's more effective.
What usually happens is that the DVLA (Driver and Vehicle Licensing Authority) holds back some of what they consider to be the most desirable plates and auctions them every once in a while. Other companies buy and sell plates they consider desirable but some of them are lame. Usually if they have to explain what a plate might mean, it's lame (for example A4GUD that they suggest might be used on a good Audi A4). One DVLA auction included the plate F355GTS which might work well on, say, a Ferrari F355GTS.
Some are little more than names (e.g. MR 51NGH) but you can also get things like DEV 1L or B1 KER. Some older plates create interesting options - I once saw a driveway with two identical vehicles parked on it with plates 2 BE and NOT 2 B. The most creative I saw was a very fast car with T13 VOM. It doesn't work until you see it in your rear view mirror.
The most pretentious I saw was a Ferrari 355 Spyder driving on Regent Street in central London with quite loud rock music coming from the stereo and the number plate 1 AM.
