You can't go wrong with Laen's service: http://dorkbotpdx.org/wiki/pcb_order
We used them for our prototypes. It's fast if you are in the USA and it's even better now as he sends out the panel once it's full. It's about 9 days from the panel ship out date to get your PCB (if you are in US). ENIG finish with purple solder mask. Sexy!
But obviously there's Seeedstudio and Iteadstudio super low cost service too. But, we have yet to try them.
Other than that, we used http://www.circuit-mart.com
when we can't afford to wait for long shipment time from US (Laen's service).