Narbacular Drop

You may have heard of the game coming out soon called Portal, which uses the Half Life Source engine to make a 1st-person puzzle game using portals that you can place anywhere, with one leading to the other. It sounds very exciting, but you don't have to wait for its release to get a glimpse of what it's like. Narbacular Drop is, essentially, the original. It was developed by students at DigiPen, and once the guys at Valve saw it, they immediately hired the students to develop Portal. It's the same idea, two portals, one leading to the other, but it's free. Even after I'm done with the puzzles (there are additional puzzles available from the site anyway), it's just fun to screw around with the portals. You can set them up so you can see yourself through the portals, or have one on the ceiling and another on the floor for infinite freefall! I highly suggest you give it a try.