Valve are giving away Portal via Steam for free till May 24. Everyone should get this if they haven't played it already, it's a 'first person puzzle platformer' where you can place two portals anywhere on the walls/roof/floor. If you jump into one of them you'll exit the other with the same speed and momentum you had when entering.
On a similar note, unless Valve are being cockteases, Portal 2 is probably coming out this year.