BatchPCB offers low cost for prototypes

BatchPCB is a service of SparkFun Electronics that provides low cost PCBs in small quantities for electronics enthusiasts.  There are some limitations and a slow turn-around with this service, but the idea is to make it cheap by batching your design with other orders. Here is a blog post about alternatives to BatchPCB if you find it too slow or need more than a few prototypes.  Here is a comparison of BatchPCB and a few other PCB Pooling services including MakePCB, and Eurocircuits. If you are new to PCB design, this article from IEEE Spectrum covers an engineer's first PCB design using BatchPCB.

You can sell your design at BatchPCB as well.  For example, you can buy a Breakout Board for the Maxim MAX3421E USB host controller designed by Oleg Mazurov of Circuits@Home.  You can find some open-hardware for sale as well, which seems like it could cause problems. One open-hardware company, AdaFruit Industries, requested one of its designs removed from the BatchPCB marketplace.