I just noticed that Steam (a video game client/installer) is now available for Mac, and for the occasion they have a special promotion on Quantz, a game written mostly in Scheme with Gambit. Quantz is only $3, which is 70% off the normal price. Another goodie is Portal, a game of logic and physics, which is FREE until May 24th (note: it is a 3.8GB download!).
Have fun!
Marc