The built-in Camera app on the Galaxy S can use the front facing camera. It's called "Self Shot Mode" and I've used it on mine once or twice, easiest way to grab a quick picture of you and a friend when there's no one else around to take the shot.
Open the camera app, press the square symbol at the top-left (the mode button), press Self-Shot, smile at the funny person on the screen!
Update November 2010: As an update, the official Samsung Froyo release has changed the way this works, there is a new icon in the top-left of the camera screen, that looks like a camera with rotating arrows around it, pressing this button swaps between the front and rear cameras. Picture of the new button to swap between cameras
See this table for a list of which Galaxy S models have the front facing camera (and other differences between the models in the range).