For one-off boards bigger than a few square inches I like 4PCB.com. $33+shipping for a 2-layer board, 5 day turnaround, up to 60 square inches for the fixed price. Minimum quantity 1. For contrast Seeed charges $4.90/sq inch for 2-layer and 5-day turn, minimum quantity 5. $66 for their 4-layer boards, though "only" 30 square inches. And they have a 1-day turnaround bare-bones prototyping service: those boards have no solder mask, but they credit the price of the bare-bones boards to your production order, so it ends up being a fast PCB prototyping service that only costs shipping). They're in the US too, so shipping is faster than the Chinese companies.
There are tons of board houses. What's unusual about Seeed is that they will do one-off assembly for a reasonable price, provided you design to their standards.
True, just mentioned 4PCB because they've had very good service in my experience (as a hobbyist/student). I've also used Seeed, they are quite nice, especially for small boards.
This reminded me of pcbshopper.com, it's a pretty neat comparison service for PCB houses. The data isn't always perfectly accurate though, especially because it doesn't take sales or other special pricing into account.
