Okay here's Mr. 737:
737-100 : Prototype, still one in operation with NASA
737-200 : Market version of the -100, very succesful
737-300 : Second generation of 737, CFM-56 engines, new cockpit(with FMC). And my personal favorite.
737-400 : Same as the 733, only longer
737-500 : Same as the 733, only a lot shorter, concurrent of the Fokker 100, only with a longer range.
737-600 : Same fuselage as the 735, but it's the 3rd build plane in the NG range.
737-700 : Same fuselage as the 733, only a bigger wing and tail, new CFM-56 engines, first plane in the NG range
737-800 : Whole new fuselage, longer and fits 158 passengers at max.
737-900 : Longest plane in the 737 range, fits a LOT of passengers.
737-900ER : Plane is to be set in service, this will be a sort of 757.
737-700BBJ : Executive version of the -700.
737-800BBJ2 : Executive version of the -800.
Legend:
B731 : Boeing 737-100
B732 : Boeing 737-200
B733 : Boeing 737-300
B734 : Boeing 737-400
B735 : Boeing 737-500
B736 : Boeing 737-600
B737 : Boeing 737-700
B738 : Boeing 737-800
B739 : Boeing 737-900
That was my 2 cents

Mees