This came out a while ago, but this fan made remix of the Portal villain GLaDOS is still awesome. You can hear version 2 and watch the video, which is a really cool overview of the game and gameplay. I also included the first version which doesn't have the video anymore, but it's still a great song c:
Portal is one of those games that's just a lot of fun, you know? even if you're not a hardcore gamer, or maybe you are and you've played it 20 times--it's still awesome! And the fan songs about the game or remixed from the game itself are just as awesome.