for the most part, an fpga is used as a stepping stone towards an asic... you would be very hard pressed to find any kind of fpga the size of a thumb-drive that is able to get a small percentage of a normal avalon chip (288 MH/s) (or even one that you can put the chip into the fpga).... for the most part, you just don't see fpga boards that small.
I should probably have clarified that what interests me is the ability to hop between multiple hashing algorithms, including currently unknown or undeveloped ones. Performance is nice of course, but a secondary goal. Obviously ASIC is not suitable for such an effort though of course the would hammer the shit out of FPGA in an 'application specific' role.
As I understand things, there are a multitude of FPGA's of different capabilities. Many devices have them, and I figure that most of them could be programmed for a lot of different duties. Some hashing algorithms are designed to be inefficient on such devices of course.
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I'll doubly-up this post and thank ~tabbek for the link to the 'mojo' board. It's interesting and along the lines of what I was thinking. I did not expect a spartan-6 to be applicable (powered by USB current at least.) It does give some info on the type of support which a spartan-class chip might require. A chart I found indicate to me that some spartan-3 might be in range from a power consumption perspective, but my knowledge of the stuff is only barely sufficient to make some rough guesses about this.