I have used
http://pcbcart.com for many years, and they are excellent. They now offer lower quality boards for prototypes, without tooling charges, and cost is very reasonable. I also use
http://www.eiconnect.com, also known as
http://zoompcb.com, and you can get double sided boards up to 5" x 5" for about $15 each for 7 day turn. Their regular prices for larger boards and larger quantities are also reasonable, and at least they are USA based. Here is an image of a board I had made by them recently: