Currently using www.insurance4carhire.com
Previously used http://icarhireinsurance.com/
For me, it's a no-brainer. If I were to pay Zipcar for its excess waiver, I'd be paying nearly 10X the price.

However, looking at the fine print, this insurance is only available for EU residents. There are some other threads on Flyertalk where people have discussed other companies that offer this kind of insurance. I understand that some US residents might get similar coverage from their credit cards.