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Actually pretty handy for doing optimized NIC/protocol processing. Particularly handy for wireless stuff IIRC. It's about as close as you can get to a software defined CPU, so the options are kind of limitless.

Learning & prototyping hardware (ASIC) design. Applications that like low latency I/O. You can respond in microseconds with real time guarantees, eg for radio link layer protocols.

Just to toss out a few ideas: software defined radio, high-throughput DSP (like audio processing or machine vision), MITMing of fast protocols, packet inspection, CPU emulation.

I found http://opencores.org/projects nothing is a killer app right now.

At work I use an FPGA as async real time controller. Not sure what utility that would have if not connected to specialized hardware.