GLaDOS is the fictional Genetic Lifeform and Disk Operating System, voiced by Ellen McLain, and featured in the PC game Portal (which also features in The Orange Box). The mechanical voice also sings the lyrics to the song "Still Alive", which is played as the game's ending credits roll. The song is actually written by Jonathan Coulton. "...Aperture begins developing the Genetic Lifeform and Disk Operating System (GLaDOS), an intelligent research assistant and disk operating system."
Excerpt from Aperture science official homepage.

Read more about GLaDOS on Last.fm.