Gaming download store Direct2Drive has begun offering game rentals, though the system isn't quite so advanced as OnLive. Rather than the games playing remotely on a server and you simply streaming the play to your PC, Direct2Drive's titles download in full each time, and then DRM causes them to expire at the end of 5hrs gameplay. It's not the only drawback with the service, either.
As well as being a little on the expensive side - $5 for 5hrs - the service only currently offers a catalog of three titles: Grid, Divinity 2, and Silent Hill: Homecoming. Still, if you later decide you want to keep the game, Direct2Drive will knock the $5 off the purchase price. We'd recommend shopping around first, but if you simply have to try a title - and assuming Direct2Drive bulk up the catalog - this could be a straightforward way of doing it.