Free

TightVNC http://www.tightvnc.com/

RealVNC http://www.realvnc.com/

UltraVNC http://ultravnc.sourceforge.net/



Commercial

TridiaVNC http://www.tridia.com/

Radmin http://www.radmin.com/radmin/comparisons/index.php

Nomachine http://www.nomachine.com