vncviewer is an Xt-based client application for the VNC (Virtual 
Network Computing) system. It can connect to any VNC-compatible server 
such as Xvnc or WinVNC, allowing you to control desktop environment of 
a different machine. 

This conflicts with tightvnc. If you need the full functionality of VNC,
(server and all) then use that build. This build is in case you just need
the client only.
