BTCMiner is an Open Source Bitcoin Miner for ZTEX USB-FPGA-Modules 1.15b and 1.15d and USB-FPGA-Modules 1.15x. The latter variant is optimized for cryptographic computations such as Bitcoin Mining and allows to build up FPGA clusters using standard components (USB cables and USB hubs).
The Software typically achieves hash rates of 90MH/s on Spartan 6 LX75 FPGA's and 200MH/s on Spartan6 LX150 FPGA's. The frequency is scaled dynamically using error measurement, i.e. the software scales the frequency such that a maximum rate of valid hashes is generated.
BTCMiner also have a cluster mode with hot-plugging support which allows to to run large FPGA clusters from a single software instance.
More details can be found on the BTCMiner home page