For low quantities of PCBs, there are two main services available that I am aware of: OSH Park and iTead Studio. They have their pros and cons, but a new kid has come onto the block: circuits.io. They use OSH Park’s facility, but have a different pricing formula. I have done some analysis to find the best vendor for the project.
Comparing PCB Manufacturer Prices: OSH Park, Circuits.io, iTead Studio and Upverter