Now here’s a cool new service. Borrowlenses.com You can rent all sorts of lenses by day, week or longer if you like. They have all sorts of lenses for Nikon and Canon. It doesn’t stop there though. You can rent camera bodies, tripods, heads, teleconverters/extenders and flashes.
I can think of several reasons this could come in handy. Want to test something out before buying? Need an extra camera body for a photo shoot? Your camera is in the shop? Borrowlenses.com should be able to fill some of these voids.