The song 'Still Alive' at the end of Portal during the credits.
GLaDOS Myspace/MP3/Companion Cube Wallpaper at the bottom of the description.
'Still Alive' by Jonathan Coulton: http://www.jonathancoulton.com
and sung by Ellen McLain (GLaDOS, TF2 Announcer, HL2 Overwatch Administrator)
I don't own the song or video in any way, nor make profit in any way. It's a great song, and I uploaded it purely based on that and to share it with others.