http://www.lensrentals.com has the largest selection (and they proudly let you know, too!) They’re also the only place in the US that does Sony alpha-system rental.

http://www.borrowlenses.com is also big, and serves up Canon and Nikon stuff.

My favorite is: http://ProPhotoRental.com

They’re pretty new, but I think I’ll be sticking with them from now on. Mostly because they are the only ones who have an online calendar-based reservation system, so you know FOR SURE that you can have your lens or body exactly when you need it. I shoot weddings, so I can’t afford to rely on a first-come first-served system.
They also have free shipping both to and from, which is nice.
On a side not, they also carry Olympus gear, which has been a real hit with a local friend of mine, who says he can’t find rental Oly stuff anywhere else.
Rentglass was definitely the innovator and I’m glad they took the plunge to start this nascent industry. But I just wish they’d do a little bit more to keep up with their competition. Perhaps with prices that low, they’re not making enough to make big changes?