For small quantities I use PCBCART (http://www.pcbcart.com). They work very well and the price is right: please take care they ask for tooling costs on the first order (not added on the subsequent orders). They have a price calculator on their web site (20 pieces 25x100mm = 2.29 euros/ea + one time tooling) - Elio.

Currently I'm getting boards made by ITead Studio (http://iteadstudio.com). Their current price is under $3 per board based on an order of ten. - Chris

Here is another small batch PWB manufacturer: DorkbotPDX (http://dorkbotpdx.org/wiki/pcb_order).