Firstly, find yourself some anaglyph (red/cyan) 3D glasses.Then, go to http://www.iz3d.com.
From here, go to "download" and get the latest drivers (use the ones that are currently Beta at time of me writing this - version 1.08. This version has a setting for anaglyph that older versions don't). Install these. You'll get an error message saying you don't have an iz3d monitor, just press ok, it still works.
Now, run the program, set it to anaglyph. Keep the program open and run your computer game.
Press the star (*) key on the number pad to turn the effect on once the game has started. Don your glasses and walla.........3D games.
The most modern games will need a good gaming computer to do this but most games should run OK. I just did it with Assassin's Creed (EXCELLENT game). Looks awesome.
If the 3D effect doesn't look quite right, click either the plus (+) or minus (-) keys to alter the red/cyan separation.
UPDATE: Check this page out to get some FREE 3D glasses so you can do this.