Client Of Win (COW) - Features client documentation and download, game information, and settings. - http://cow.netrek.org/
Continuum Software Library - Check out the section 'blessed clients' for a list of many clients for different platforms. - http://www.continuum.real-time.com/software.html
Netrek Vanilla Server - Here you can get the latest source of the Vanilla Netrek Server. (NB: to play you don't need the server, just a client for your platform). - http://vanilla.us.netrek.org/
JTrek - A Java Netrek client - Beta - http://www.starwave.com/people/robertt/JTrek/