The guys at NetAdminTools.com have put together a very detailed article describing how to build your own Cat5 cable tester.  When I say tester, I mean a VERY primitive tester, its not going to tell you how far down the cable your kink is or anything neat like that.  The article covers everything from Cat5 wire configuration to using assembly code to design a circuit.  While, from the looks of it, this thing does nothing more than two link lights and the ping command would, one could really learn a lot from building one of these.  Nice work guys!