At work we use them. There is a small plastic module that has USB and monitor connections at one end then the connection for cat5 at the other. Basically it lets us use a length of network cable to run our KVM switches. As far as I know it doens't do anything special, like actually connect to the network if you were hoping for that.

(could be wrong on that, but we don't use them that way if they do work like that)
Found an Ebay auction with one similar (PS2) You can see where the cable would connect, so it would extend as far away as your cable would allow. They probalby just choose to use that instead of a proprietary cable since it's plentiful and cheap.