You could submit this as a seperate deal if you like, but the best (price-wise) game rental service I've seen so far has been gameznflix.com, with prices clocking in as follows (non-trial too, I might add)

8.99/month - one game
12.99/month - two games
16.99/month - three games

I've used Gamefly, and they're pretty decent as well, with two or three day turnarounds and the stacking rewards system (which isn't great, but it's better than nothing).   I also think Gamefly's a hella great way to buy just-released games for below retail, since you're getting a "used" game that's usually in perfect condition for around $10/15 off.  They just mail you the box.

I haven't bought anything from Gameznflix, so I can update if I decide to (might buy Splinter Cell).