None, unless you plan on developing your own bitstreams. And then, when you develop your own bitstream not much, because this is an old Spartan-3, not the newer Spartan-6 that is used by almost everyone else.

Great educational toy though. XC3S400 is supported by the free Xilinx ISE Webpack.

Edit: I take back the "great" from the "educational toy" above. I took $127.15 as a total cost, but this is just postage. So the better description would be "great ripoff on an educational toy".