www.gametap.com
If you haven't heard of Gametap before, it's sort of an "all you can play" online video game rental service. With games from tons of systems (ie Genesis, Neo-Geo, Dreamcast, PC, and they've started introducing original games too, like Sam & Max, etc.) For a monthly price, you can play anything in their catalog, as much as you want. It's by Time Warner (Turner started it a year or two back).
It's always been a good deal, but now they just introduced a FREE section! They have around 30 games out of their library of (currently) around 900, and apparently the selection changes every week.
The only "catch" is there's a banner ad along the bottom of your screen when you play the game.
I plugged in a controller, and I'm playing Mutation Nation on my TV right now (they've even got Tomb Raider: Legend up for free this week). The Gametap interface is slick, can be navigated by mouse, keyboard, or control pad.
I'm going to sign up for the full Gametap service eventually, but want to get through my backlog first.