I used PCBCart in the past. They have a nice online pricing tool and they also are pretty good at spotting any design issues that one might have in the board design. They have a large capability spectrum. They are not ultra cheap but they are reliable, encourage good pcb design practices and the PCB quality is on par with the price.
For prototypes I use OSHPark, they have great balance between price/speed/reliability. The PCB quality is extraordinary, much better than PCBCart or any other low cost prototyping service I have tried so far. (maybe Eurocircuits is comparable, but the prices usually are much higher for similar lead time, and I am now located in the US so they are not really an option anyways)
Recently I also make my production runs through OSHPark too. Unless I need some crazy 8layer board with blind via I will likely continue using them for all my standard spec boards. Additionaly they are a very communicative and helpful bunch. If you have some special requirement email@example.com is very quick at doing what they can to help out.