Named Game of the Year by over 50 publications, Valve's début title blends action and adventure with award-winning technology to create a frighteningly realistic world where players must think to survive. Also includes an exciting multiplayer mode that allows you to play against friends and enemies around the world.
A compilation of individual 3D environments built by Members of TWHL a site for Half-Life Mapping Tutorials and Resources).
I've seen this before somehwere. It was amazing.