There should be an ilegall expansion set called darkside of the swordcoast, and its availlible at \
http://www.teambg.com/ . Though I never played it I've heard its pretty good. I just dont download it because I dont really trust ilegall add-ons or trainers. They can pretty screw up your game. So just backup your BG before you install it !!!
Also there are ( IN case you never heard of it, wich I can hardly imagine) Icewind Dale, Planescape torment, Neverwinter nights (not released yet) and ofcourse Baldurs Gate 2.
IWD is what I like to call BG 1,5. or more BG 0,5 cuz it takes place hundreds of years before BG. Its a pretty good Spinn off. NWN nights is a TFR game meant for MP. Planescape Torment looks a lot like BG, but hasnt got annything to do with it. And Bg 2 is well.....
Just some stupid game. You shouldnt buy it.
greets.