Three different radar clients are currently available for usage on the VATSIM network:

• ASRC
• VRC
• Euroscope

The latest version of ASRC (v1.2) was released in Nov 06, and is available for download from: http://library.avsim.net/download.php?DLID=93340

VRC, the other radar client, was updated to version 1.2.1 in August 2007.
It is also designed for multi-monitor use, but many use it single screen with success.
VRC can be downloaded from:
http://www.metacraft.com/VRC/download.shtml
Be sure to read the VRC documentation to fully understand the differences between ASRC and VRC.

Our youngest VATSIM Controller Client is EuroScope.
The first public version was released in late 2007. It is inspired by solutions and feedback from real-life European Air Traffic Service providers, with long on-line controlling experience from VATSIM.
Euroscope can be downloaded from:
http://www.euroscope.hu/main.php

Please note that the VATSIM forum includes separate topics on all of the above Controller Clients. See the forum, and click on the client software you are interested in for more information, as well as benifiting from other user's experience.