The Nexus Client is how players access the game of Nexus. It is downloaded from the NexusTK website, and should not be obtained from any other locations. All updates to the client should also only be obtained from the official Nexus website, for the security of all players.
Never download any programs sent to you by other players, as they are often hacking programs or viruses in disguise.
The current client in use is version 6.5, released by Kru Interactive on October 12th, 2005.