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.
(4 maps) Their former website is no longer reachable, but the demo they left behind is a great opposing force styled story for half-life.
btw. their intention was singleplayer and multiplayer :)