Some time ago, I mentioned a PC buy-back program between Greensight and Costco, where Costco would issue you an in-store credit for the value of your old PC's. Apparently, that program is no longer in effect, however, I did find another program run by IBM.
IBM will buy back your PC's, servers, printers, etc. They can be either IBM or non-IBM. If the equipment has no value, IBM will safely and securely take care of the disposal. If you want more details, I've included a link in my GoGreen list, but here it is again: IBM Buy-Back