I could litter this post with cake and companion cube jokes, but let's just get right to the meat of it...
Portal is free, for a few days. If you don't already own a copy, GET IT NOW. Heck, even if you do already own one, it wouldn't hurt to have a...uh...second Steam account? With your second copy of Portal in it!
It's OK. You'll feel good about this. Click the button and get the game. Even if you're not sure that you'll ever, ever play it. Just get it. It's free.
Get Portal. There is no good reason not to.