Unknown to most users Leopard actually has a VNC client build-in which is used by the Remote Desktop and its Screen Sharing applications. It turns out that you can do a Command-K in the Finder, which will open the “Connect to Server” window, and use a URI like this: vnc://hostname to connect. If the connection is not running on the default VNC port of 5900 you can add the port in the usual way such as vnc://hostname:portnumber. If you saved the URI as a Favorite Server just click on Connect button: 

A Authentication window will popup for you to enter the required password. Click the Connect button to continue:

You may be presented with the following Warning Window depending on the VNC server you connecting to:

Just click on the Connect button to finally be connected. Depending on the Host you connected to you may have to login to it:

To exit (quit) you can either click on the red circle (bubble) in the top-left hand corner of the VNC window, or by pulling down the Screen Sharing menu and selecting the Quit Screen Sharing option:

You can also open Screen Sharing directly. The Application is located in the /System/Library/CoreServices folder:

You can drag the application icon onto the Dock to allow quick access to it:

When using the Screen Sharing application the following window will open:

Enter for your Host URI and click the Connect button to get connected.

Next a Authentication window will popup for you to enter any password required. The next steps will be the same as if you had used the Finder’s Connect to Server menu option (see above).

Personally I prefer the “Connect to Server” route since you can save the URL as a “Favorite” so you don’t have to reenter the URL every time you want to connect.