This past weekend I downloaded Portal from Steam for free (still available free of charge until the May 24th), but have yet to play it. I also came across a site that has a browser based Mario Bros game, titled Super Mario Bros.2 Star Scramble. It’s a fairly decent game, though it can be a pain playing this type of game with a keyboard instead of a controller.
Also, if you visit SuperMario.com, they have a ton of browser based Mario games to try out.