If you dont have your own wet-etch system, you'll need to send pcb's out to be made by a company

Many companies have a "quick prototyping" option, where you get your board back within the week, but sometimes without soldermask or silkscreen. Most hobbyists use this option to make small runs of boards.

Advanced Circuits (4pcb.com) aka Barebones PCB aka 33Each.com aka FreeDFM.com
These guys have a bunch of different options for cheap boards. The fastest is barebonespcb.com, which will get your boards shipped next day, no solder mask, etc. for $35 and a sq. in. cost. This seems to be one of the cheaper method available online. I've used it a lot and have no complaints. Since it's square-inch priced you can panelize (put mulitple designs on one PCB and cut it with a saw your shear)
Another method, if you're a student, is their $33 deal where you can get a 60sq.in. board for $33 ($50 after shipping) with soldermask and top silkscreen. If you're not a student, theres a 3-part minimum, so its $100. Still, a good deal if you want soldermask. If you want to panelize this design they'll add $50.
They'll rip you off on shipping, so be sure to calculate that into your total!

Alberta Printed Circuits (APCircuits)
I like their quick turn P1 service, I dont like their weird drill-size constraints. Apparently theres a quick way to deal with this in Eagle but I dont use them much anymore

Sierra Proto (2justforyou)
The URL is kinda retarded, their DFM production staff seems so-so compared to 4pcb, and they add a $20 'service' charge to their proto PCBs which is really obnoxious (just be honest about the price, is all I ask). But when they get around to it, the quality of their "NoTouch" boards exceeds that of 4pcb w/gorgeous silk-screening. Whats really nice is that they allow much larger PCBs for their prototype runs, 80 sq. & larger.

PCB 123/PCB Express
The software is lame, but the service is pretty good, apparently. I haven't had time to try it out though. Seems to be pretty much identical to Advanced Circuits in pricing and structure: Cheap (no silk/mask) next-day PCBs, a couple-day prototype service with green mask and white silkscreen, and then a more detailed 'production' service. You can check out their pricing here.

Gold Phoenix
This is the company Sparkfun uses for their cheap PCB service. You can get 1000 sq cm of PCBs for about $100. Yow! Check out Julian Bleekers very nice review. I order prototypes from them, but for production be sure to go with electrical testing as I've had a few flawed PCBs (vias that dont connect thru, etc)

E-TekNet
A front-end for some PCB company in China. A friend recommended them but afer making 500 PCBs there, I found their customer service to be frustrating and I was unhappy with the PCBs I purchased. I wouldn't use them again.

Olimex
I've never used these guys, they're in Bulgaria so things take a while to get to/from them if you don't live in Europe. They're dirt cheap, though. I've heard some good things about the quality.

BatchPCB (SparkFun Prototyping)
They have a 'prototypers' service, $10 + $2.50 per sq in. They use Gold Phoenix as their downstream provider, its probably good for most small projects but be aware that it may take 3 weeks to get your PCBs (they are low priority and made in China)

OurPCB
Steve Conway recommends them after a PCB run. Made in China, but with reasonable service and shipping. Probably similar to Gold Phoenix in quality.

PCBCART
Some of my friends use their service but I've pretty much decided they are probably a bad scene after they used a picture of one of my PCBs as their advertising without attribution, permission or (at least) manufacturing such a PCB! Upon my polite request they use a different image they did not respond and the magazine in which the advertisement was placed also didn't do anything about it. Kinda lame in my book.