There's quite a good Wikipedia article on this.
Amongst all the quiz and reality shows, there are a few intriguing obscure nuggets:
- The Golden Girls is an adaptation of a UK TV series, The Brighton Belles.
- They've tried to adapt Fawlty Towers three times.
- Not the Nine O' Clock News was adapted for the US.
- One Foot in the Grave became Cosby - presumably the British version didn't have enough (ie any) rapists.
- They tried to adapt Spaced, IT Crowd and the Young Ones but they failed at the pilot stage. This is probably A Good Thing. Although I might see if I can dig out the pilots.