So you've downloaded Steam for Mac and have taken advantage of the fact that Portal is free for a limited time to finally try this awesome little game. If you've never played Portal before, you might not notice that there's a slight problem with the Mac version's portals. You're supposed to be able to see what's on the other side of the portal through the portal. This is both a fun visual and an important tool in solving some puzzles (though I believe without it the puzzles are just harder, not unsolvable).

The black portals on the Mac are a bug, but an easy one to work around. Start the game in "windowed" mode, then go back to full screen. To do this, set the game launch options to include -windowed. (Source.)

PC users: Portal is free for you too!