PCB Manufacturers
The following is a user-submitted list of PCB manufacturers. Your results may vary.
BatchPCB
2.50 / square inch, 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.
Advanced Circuits
Good online design rule checking, easy web ordering interface, and small standard trace/space tolerances. Students can order single PCB's for $33 each.
Olimex
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.
PCB123
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.
PCBFABEXPRESS
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.
Other fab lists
Other web sites with long lists of PWB fabricators:
