Screenbond 1 and 2 layer, low tooling cost and cheap for standard turnrounds, can do 24hr turnround.
Stickleback They have a prototype service via their Ebay shop
PCBTrainOnline PCBs and assembly service
PWTFull range of PCB services, including flex and aluminium
PCB PanelFlat-rate panel service
Quick-teckPCBs up to 500x630mm
SMT StencilPolyester stencils
TecanStainless stencils, screening cans and photochemical etching
Spark LaserStainless stencils
PCB TrainStainless stencils
Solder ConnectionStainless stencils
Advanced Circuits Full range of prototyping/production services based in Colorado; popular amongst university students.
Australia / New Zealand
http://www.pcbzone.net/ PCBZone - Oz/NZ based panel sharing proto service.
http://www.circuitlabs.co.nz/ Circuit Labs. The commercial arm of PCBzone above. For bigger qty. Made in NZ.
http://www.acetronics.com.au/ (simple PCB's, assembly, silk screen NC mill etc)
Online - Worldwide
PCBCart Long established Chinese supplier, with wide range of options at good prices, even after shipping costs.
Eurocircuits Wide range of prototype and production services including aluminium, also stainless solderpaste stencils.
PCB-Pool Pooling service, with free stainless paste stencils.
ITead Studio Very cheap two layer prototype service.
Seeed Studio Cheap two layer prototype service with thickness and surface finish options.
DorkbotPDX Two and four layer pooling service.
PCBwing Similar to PCBcart online proto service.