The program is called eDonkey. Both the command line client and the GUI can be downloaded
here.
Good luck.
By the way. When you've installed eDonkey, firefox and konquerer will still tell you that ed2k is an unregistered protocol. I haven't yet figured out how to fix this (in Windows, clicking the link will automatically start the download in eDonkey). But you can just drag and drop the link from firefox into the eDonkey GUI.