I always know there are a lot of people out there way smarter than I am (a lot) but I'm still amazed after seeing Portal - Advanced Chamber 18 done with 0 Portals. Check out the 'related videos' as well for other similar accomplishments. These guys do things like lean security bots against doors to keep them open when they open, exploit little bugs in the HL2 engine to climb walls, and other such coolness. Very impressive!