If you own a GeForce graphics card and still haven't played Portal, then you might want to grab this Nvidia-exclusive download of Valve Software's 2007 hit first person shooter. Portal: First Slice is freely available via Steam, which will automatically scan your PC to check if you've got an Nvidia card under the hood.
Unfortunately it is not the full game but rather an "extended demo" - as the website calls it - and offers no new content to those who have already played the full version of Portal. The demo comes as part of a recent agreement between Valve and Nvidia to "collaborate on development, distribution, events, and more." Additionally, Nvidia users can also download Peggle Extreme, Half-Life 2 Deathmatch, and the Half-Life 2 tech demonstration level "The Lost Coast" free of charge from that same site.