The following is a user-submitted list of PCB manufacturers. Your results may vary.
Best prices around for >2 layer boards. 4-layer boards up to 30 sq in. with 7/7 are $210 for 5, including shipping. Turn time is about 10 days including DHL from China. Service is not as fast as bigger outfits like Advanced Circuits, but still fine.
2.50 / square inch + $10 setup, must use your own software to generate Gerber files. Easy-to-use web ordering interface, but slow turnaround time -- 10-20 days. Boards come out very nice -- precut, double-sided with silk screen and solder mask on both side.
Good online design rule checking, easy web ordering interface, and small standard trace/space tolerances. Students can order single PCB's for $33 each.
Bulgarian PCB manufacturer - $33 for a 6.3" by 3.9" board. I.E. cheaper than BatchPCB, but with a minimum board size requirement, silkscreen on one side only, and a more complicated ordering process.
Offers their own free (proprietary) schematic and PCB layout software -- easy to use, but you are locked into using their service. Integration with ordering system simplifies the process of ordering boards.
5 Boards for $60 ($12 ea. for 10 sq. inches) with a 5 day turn time. Good quality work, but turn times can be delayed if you have to correct any issues that don't meet their specs.
2 layer PCB's with next day shipping for under $50. Online quote system and instructions for creating Gerber output from most popular PCB programs.
4 2 layer boards for $18.99 each. Great prices for production board runs. Helpful staff.
Similar to PCB123, in that they have free propietary schematic and layout software, but their Mini-Board service (3 identical boards, 2.5" X 3.8", no silkscreen) is priced well at $51 total, which includes shipping. If you need small boards quickly, this is a good choice, since they ship the next business day.
Fast turnaround (~8days + shipping), instant online quotes. $30 tooling fee, under $2/sq in.
Other fab lists
Other web sites with long lists of PWB fabricators: