Game Fly
Man that site is awesome.
You pay 21.95 a month.
You get unlimited rentals per month.
2 at a time.
It cost nothing to send them back, they send with the games envelopes so you can send them back without charge.
And after you are done playing, you can buy them at discount prices. I bought Jak 2 for 29.99 and Castlevania for 34.99.
And they are always giving deals on the games you rent. They lower the prices quite often.