TightVNC

TightVNC is a free remote desktop application. With TightVNC, you can see the desktop of a remote machine and control it with your local mouse and keyboard, just like you would do it sitting in the front of that computer.

How secure is TightVNC?
Although TightVNC encrypts VNC passwords sent over the net, the rest of the traffic is sent as is, unencrypted (for password encryption, VNC uses a DES-encrypted challenge-response scheme, where the password is limited by 8 characters, and the effective DES key length is 56 bits). So using TightVNC over the Internet can be a security risk. To solve this problem, we have plans to implement built-in encryption in future versions of TightVNC.

Compatibility
TightVNC is cross-compatible with other client and server implementations of VNC; however, tight encoding is not supported by most other implementations, so it is necessary to use TightVNC at both ends to gain the full advantage of its enhancements.

RemoteVNC
RemoteVNC is a fork of the TightVNC project and adds automatic traversal of NAT and firewalls using Jingle. It requires a GMail account.