The first time I played Portal, I did it in two sittings. I think my children had to eat or something, but their tiny collective voice was about all that could break my concentration from this game. With the exception of the Bard's Tale series, to which I also lost much of my life as a youth, Portal is the most engaging game I've played on any platform.
Steam for Mac is finally out, and Portal is available for free for a few more days. Go download and get this fascinating puzzler. You have to play through it before Portal 2 hits the shelves.