I was thinking about this for quite a while. We like to strip parts from old boards both to save money but also to find those "cool" parts that aren't really accessable in the low volumes the average experimenter can muster. Any way one step away from this is merely to reprogram that hardware and leave it in situ on the board. In practice that board has then become a cheap alternative to a manufacturer's dev board.
In the case of FPGAs this is obviously easy to do as long as you can get to the programming infrastructure or at least the Flash with the bitstream on it.
I was suitably impressed to find that some one had actually done this and not only that had used the device to do some Bit Coin mining!! 
His Article is here and well worth a read
Here's a link to the actual FPGA BitCoin project used to test the device.
Anyway I've seen some scrap boards with some quite large Virtex parts on them so I will definately be taking another look now!!
Love your scrap bin...John