Who will make PCBs for hobbyists?
A selection of names which came up at http://groups.yahoo.com/group/kicad-users/ in November 2011. All of these fab houses therefore probably accept Gerber files, however they might have been created.
Remember that any of these could have been posted by the board maker themselves, just trying to drum up business!
Sadly, http://batchpcb.com/ is no more. See https://www.sparkfun.com/news/1138 for details. In that, Sparkfun suggested (May, 2013, but post still present Oct 14) that we use https://oshpark.com/ (These notes, unlike the others here, are from the page's editor) BatchPCB offered the usual, plus a rather fun twist… if you wish to make your board available to others, just say so. (It is optional… you don't have to.) Your board would then be available from the BatchPCB website, with no hassle for you, apart from answering customer questions! I couldn't find any information on what part of the price they collect they will share with you. OSH Park seem to have something similar under the tab "shared projects". BatchPCB started as an offshoot of Sparkfun Electronics, a supplier to hobbyists which I have used happily for several years. It seems very "newbie friendly"… but don't abuse that, and push up everyone's costs. Read the "how to do it" information before you ask questions!
Other websites, presented in no particular order. The "information" after any link is just an edited version of what the poster said… I cannot vouch for it. Caveat emptor! This list will, at least, be worth what you paid for it!
http://www.pcbunlimited.com/pcb-panel-share-p-725.html (March '16: minimum order: $5. Two layer boards @ $1.65 per sq. inch. Many other options.)
http://www.pcbcart.com/ It is China, but works really well.
http://sunstone.com/ Mulino, Oregon, USA
http://www.pcbuniverse.com/ A broker getting boards from elsewhere
http://www.seeedstudio.com/ From the home page, go to the "Services" section. Their "brand" is "FusionPCB"… for which the home page search box will not give you hits.
http://www.sfcircuits.com/pcb-production-capabilities/hobbyist-special have a hobbyist special… $199, at 10/14. If you visit the site, you will find details of the specs they can work to for you. Some firms are too proud to bother with hobbyists… we will, after all, probably make some mistakes when we submit files…. but San Francisco Circuits seem willing to take the chance… please try not to abuse their generousity. Make your submission "professional".
http://www.cyindustrial.com/price.html is an interactive page where you can get a quote for a specific job. If, say, you wanted five 2 layer prototype pcbs size:≤100mmx100mm, then your price would be US$48.00, plus less than $US10 for economy air mail from China (7-15 days), November 2014. (They also ship by faster routes if you want to spend more.) Accept many alternative forms of payment.