Very little really, although the 3GS IS 0.07 ounces heavier

3GS has both video recording AND video editing function, you can't do this with a 3G
The 3GS has an integrated magnetic compass and a Nike support app built in, the 3G doesn't.
The 3GS has a glossy back panel, the 3G is more of a Matt finish
The 3GS runs at 600MHz, the 3G at 412 MHz
3GS has double the wireless speed of the 3G
3GS has a 3Megapixel camera, the 3G is 2
3GS runs OpenGL ES 2.0, the 3G uses OpenGL ES 1.1 (graphics software)
I think the battery lasts a bit longer in the 3GS as well, but this varies phone to phone and depends heavily on which features you have turned on.