I actually finished Portal’s achievements a long time ago (some of there are really hard, especially the challenge ones), but then, just prior to releasing of Portal 2, VALVe added a new achievement called “Transmission Received” with a cryptic hint “..?” All you have to do is find 26 radios hidden throughout the game and deliver them to specific locations. I couldn’t get myself to doing it, because I knew it would mean to play the whole game from the beginning to (almost) the end. But today I finally started it up again and after 3 hours I got the achievement, so I’m at 100% again. Yipee!