Before The Big Bang Theory became yet another Chuck Lorre masterpiece, the original pilot was rejected by executives.
The initial pilot featured Sheldon and Leonard, played by Jim Parsons and Johnny Galecki, sans Howard and Raj. The female lead, Katie, was played by Amanda Walsh, and the boys also had a nerdy female friend Gilda, played by Iris Bahr (Blossom).
While the premise was the same --- centering around two genius roommates -- there were some big differences. Sheldon wasn't nearly as uptight, Leonard was a much better dresser, and Sheldon even had sex! It reminded me more of Two and a Half Men then Big Bang.
The original unaired pilot has never been released, but we were able to find a snippet on YouTube.