A couple of the biggies: Avelle (www.avelle.com) and From Bags to Riches (www.frombagstoriches.com), which rent designer handbags, and Borrowed Bling (www.borrowedbling.com), which of course focuses on jewelry. Most of these sites have a range of membership options that allow you to rent at a discount, or you can rent as a guest for full price.
Before you sign up, a few things to think about: How often are you going to rent items? Are you renting something that you'll use a lot, in which case, would buying it actually be more economical? Is insurance included if the item is lost or damaged? Sometimes it is, but often it's not. The last thing you need is a big bill when you're trying to cut back.