I'm usually a fan of SeeedStudio but recently I've seen some comparisons between these three services (SeeedStudio, iTeadStudio, OSH Park), and OSH Park seems to have much better quality.
Read about the comparison here: http://www.kickstarter.com/projects/paulstoffregen/teensy-30-32-bit-arm-cortex-m4-usable-in-arduino-a/posts/305527
My next project might actually use 6 mil trace and spacing and 13 mil drills, I might need to use OSH Park to guarantee that the PCB actually works.