Some things FPGAsRus has done with FPGAs:
Super Computers: Controlling the processors and the interconnect fabric.
Quantum Computers: Control of the photon generation and detection, coordinating and synchronizing local and distant events. Processing results into the data.
Pseudorandom Binary Sequence (PRBS) Generation and Check: Up to 43 Gb/s (128 bits at 333Mb/s) for multiple patterns.
SONET/SDH OC1 to OC768: Line/Section/Path/payload generation and verification.
IP protocol: When trading stocks on the stock market, first one back with a response to an offer gets the sale. Using hardware to speed the response can mean a big profit for the stock broker.
Embedded: While FPGAs are great for pure speed, there are some tasks that are better left to a processor when speed isn't such a requirement. Like setting up a TCP link over the hardware IP link with the Xilinx Embedded development kit.