Portal and Portal 2 is orignally a 3D game by Valve. This free flash version of the game, captures the concept from the original game quite well, and gives you a very nice combination of FPS and puzzle gaming.
Most of the levels are quite easy, once you figure out how the portals can be used. The last 5-10 levels are much harder. Below is a video walkthrough on how to complete all levels 1-40 i Portal: The Flash Version.