There are better ways of course, but you can also use VNC by adding the following values to the VMX file of each VM you want to enable it for:
RemoteDisplay.vnc.enabled = "TRUE"
RemoteDisplay.vnc.port = "5900"
RemoteDisplay.vnc.password = "vncpassword"
You will need a VNC client such as UltraVNC http://sourceforge.net/projects/ultravnc/, then you just connect to the esxi host on port 5900 (in this example).
Tested on ESXi 4.1. Version 5 may require firewall adjustments.