Transpact.com has the lowest fees, depending on the currency you are using. And it is also one of the most flexible and professional escrow services.
You can try it in Moneybookers as well. Moneybookers have a option to escrow money and there is no fee for that.
Elance.com and freelancer.com offer the best escrow services and keeps you on safe side while selling your services online. I have used both of the sites and found these sites quite helpful and responsive on escrow disputes.