Wikipedia has a good list of the different versions. The article also goes over many of the differences at a high level.
My favorite version is probably 2nd ed AD&D pre-Skills and Powers. It had some of the balancing factors which 3rd lost while remaining fairly simple at heart.5. Can anyone give me a general feel or idea about these systems to help me decide if I'd like them?
As for whether you'd like it - I suggest taking a good look at Castles and Crusades - it's an attempt at keeping the best parts of d20 while reverting other areas to something nearer AD&D.
Edit: Almost forgot - there's also OSRIC, Old School Hack, and Swords and Wizardy.