Virtual Network Computing (VNC) is a graphical desktop sharing system that uses the RFB protocol to remotely control another computer. It transmits the keyboard and mouse events from one computer to another, relaying the graphical screen updates back in the other direction, over a network.
Accessing VNC Servers from Mac OS X is very easy, you dont need any 3rd party applications.
In the Finder select "Go / Connect to Server" or just press CMD+K.
As Server Adress enter vnc://the-server-you-want-to-connect-to and press "Connect"
And thats it, your connected with a quite decent client, which supports all basic VNC features, a great OSX integration and even a Lion Fullscreen Mode.
Tip:
If you get an encryption warning, just ignore it.
If the VNC Server is password protected you can enter it now.
Tip:
On OS X you can connect to all kind of services just using the Finder, this includes:
afp://, aat://, file://, ftp://, http://, https://, nfs://, smb://, cifs://, or vnc://."