God of War III at GDC 08

[Shin-Ra]

The Wikipedia entry puts it better than I could:

The Game Developers Conference (GDC) is the largest annual gathering of professional video game developers, focusing on learning, inspiration, and networking. The event comprises an expo, networking events, awards shows such as the Independent Games Festival and the Game Developers Choice Awards, and a variety of tutorials, lectures, and roundtables by industry professionals on game-related topics covering programming, design, audio, production, business and management, and visual arts. The next conference will take place at the Moscone Center on February 18-22, 2008.

A developer may show a released game to demonstrate an approach/technique that lead to success or failure.

Sometimes you'll see something brand new like home demonstrated because home's success is partly reliant on first and third parties (in the audience) supporting it with their own content. LittleBigPlanet demonstrated innovation and again, perhaps even an opportunity for third parties to produce their own content/levels that would be downloadable to the game to promote their own.

You just need the remember the event's aimed primarily at developers so don't expect a ton of new stuff to be unveiled for our benefit.