Here's a list of some popular 3rd-party VNC Add-on's:

Fastpush is a Windows-specific script that can be used to remotely 
install WinVNC onto a large number of computers. These target
computers must be running NT, 2000 or XP, and you need admin rights 
to do this, but if you're looking for the best way to install VNC
into your whole LAN, this is it.

You can get a free copy here: http://www.darkage.co.uk/

With this tool you can manage your VNC connections.
The features are:

- - BuiltIn Viewer with support for Tight Encoding
- - Network Scanner to find VNC Servers on you LAN
- - Monitor your computers - so you can see whether a 
    host is online or not.

VNC Admin Console runs on all Windows Plattforms and is 
totally free for personal use.

You can download it on http://www.mast-computer.de

From the website:

"VNCon is an easy to use remote administration tool which allows you to 
connect, view and take control of any computers running VNC (Virtual Network
Computing) on your network. The list of computers can be obtained using 
various methods, either probing your domain, scan a set of ip ranges, or 
entering each computers details manually."

VNCon is Windows-only and not open-source. The IP scan for VNC is purported
to be very fast and very flexible. You can get a copy here: 

http://vncon.chronetal.co.uk/

VNC Commander provides a multi-connection manager with a 
builtin VNC viewer. Windows only, freeware:

http://www.amsoftwaredesign.com/vnccommander.asp

From the project's maintainer:

----------------------

VNC Manager is an easy to use remote administration tool which allows you to connect, 
view and take control of any computers running VNC or Windows Terminal Servers. It 
provides an easy to use intuitive GUI interface and unique features that can significantly 
improve the user productivity of network administrators. The list of features includes but 
is not limited to :

 - Latest RealVNC 4.0 and TightVNC 1.3 distributions are fully supported.
 - Thumbnail View allows multiple computers to be monitored on-screen at once.
 - VNC Manager has built-in support for HTTP and SOCKS5 proxies.
 - IP address and Windows domain/workgroup scan. Allows scan for active VNC servers.
 - Server Status scan. Checks from VNC server status. Can be started manually or set to update 
    server status via time interval.
 - VNC deployment wizard. Allows remote installation of TightVNC and RealVNC based servers.
 - External Tools. Allows to start an external application.
 - Can connect to Windows Terminal Server with a nicely integrated full featured built in viewer.
 - Start/Stop/Restart the remote Windows service.
 - Network administrator can reboot or shutdown remote computer.
 - "Reset Remote VNC Server Password" wizard. Allows you to mass reset password on a remote
    VNC servers.
 - Capture and save screenshot of remote VNC server screen.
 - VNC and RDP viewer window stretching.
 - Automatic search folders provide a quick filter for connected VNC and RDP sessions.
 - Send messages to other computers running Windows NT/2000/XP.
 - Default session. Allows to set default setting for newly registered servers.
 - Default password. You can specify a default password to connect with, or a specific one for 
    each connection.
 - Active sessions can be viewed in the separate windows or in Tab view panels.
 - VNC Manager does not require VNC Viewer to be installed. It uses an ActiveX VNC Viewer 
    control developed by SmartCode Solutions.
 - VNC Manager can send Wake-on-LAN "magic packet".

--------------

It's Windows-only and not open-source. You can get it here:

     http://www.s-code.com

From Steve Bostedor:

VNCScan was the first scanning and management application for VNC.  Since
version 1 was released in 1997, TGCS has been committed to innovating new
features that save you time and frustration.  This is a $30 shareware
application with full commercial support.  The free Microsoft DotNet framework
and MDAC 2.7 are required.

With VNCScan, you can:

* Scan up to a class A address space
* Check the status of a single computer without a rescan
* Scan multiple ranges per group of computers
* group scan ranges into logical structures
* Remotely deploy VNC to NT/2L/XP desktops with FastPush script
* Configure the FastPush script with a GUI
* Start and stop the remote VNC service
* Reboot the remote computer
* Ping the remote computer
* Export computers to HTML
* Check the version of VNC on enabled clients
* Find who is logged into enabled computers
* Connect to Windows file shares on remote computers
* Move computers (drag and drop) between groups
* Manually add computers to groups
* Keep track of status of computers with visual effects

You can get a copy here: http://www.vncscan.com

This program uses VNC to create a dual-monitor effect. So if you have two 
or more computers on your desk and you are tired of having several keyboards 
and mice around to control them, this is the program for you. While running 
this program, you can move the mouse pointer beyond the right or left edge 
of your primary display and the pointer will appear on the other computer 
screen. If you have ever used x2x or a computer with two graphics cards, you 
get the idea.

It's open-source and available for both Windows and Unix.

Windows: http://fredrik.hubbe.net/win2vnc.html
Unix: http://fredrik.hubbe.net/x2vnc.html

Kaboodle is a LAN Management and LAN-to-LAN connectivity tool that
builds upon VNC's capabilities. With Kaboodle, you can:

* quickly and continously discover all VNC Servers on your LAN
* remotely install VNC onto PC's that aren't yet running it
* securely tunnel your VNC connections
* securely transfer files 
* setup a LAN-to-LAN connection, and then access all of the VNC
  Servers on your partner's LAN

Kaboodle is free, open-source, and Windows-only. Get it here:

                http://www.kaboodle.org

Note: Kaboodle is a product of Echogent Systems, Inc., which hosts
      this FAQ-O-Matic website for VNC.