OSH Park (formerly known as Laen's DorkbotPDX group PCB order) is very popular for prototypes in the US maker community (look for the trademark purple PCBs), and at $5/inch^2 with free US shipping it's cheaper than batchPCB for smaller boards, in addition to getting 3 copies of the board for that price. It's very high quality and it's all done at a US fab house with the 2-layer batches sent out every 2-3 days and then shipped US Mail, so if you're in the US it's pretty fast for the price.

And for pure price, Seeed and ITead (both are rumoured to use the same PCB fab) are very popular with their $10 for 10 copies of a 5cm*5cm board service. The quality is reportedly much more variable with these guys, and the shipping takes a lot longer to reach the US, but you can't beat the price. For the rest of the world, the shipping from China is probably cheaper and faster than from OSH Park, so unless you need the quality of OSH Park this is probably both cheaper and faster.