While Mac OS X Leopard includes a built-in VNC client (as well as a built-in VNC server), they do not always play nicely with everyone else's VNC server/client. However, they play VERY NICE with other Macs running the build-in VNC client and server.
Mac OS X Leopard's client and server do seem to be very sensitive to communicating with other VNC software that is not working with Millions of Colors (24 bit color).
I would actually suggest using a 3rd party VNC client on your Mac.
First: JollysFastVNC - Very good and frequently updated. I like it.
<http://www.versiontracker.com/dyn/moreinfo/macosx/33113>
Second: Chicken of the VNC. I use this as well.
<http://www.versiontracker.com/dyn/moreinfo/macosx/14099>
Those 2 are the top choices. There is also RealVNC, and you can use MacPorts.org or Fink.sf.net to install an X11 version of TightVNC (I use TightVNC at work connecting to a RedHat Linux box - gives me F9 boot time capabilities).
Depending on the situation, I use the Built-in VNC client/server when talking to there Macs, I use JollysFastVNC on my older Mac OS X Tiger system (does not have a built-in client), I use Chicken of the VNC to talk to my Mom's Mac OS X Tiger system across the internet, I use TightVNC at work connecting to Linux. For casual connections, I may use any client to connect to any other VNC server depending on my mood, and which client has the server connection information already configured and saved.
NOTE: I have also been a long time user of Vine Server (aka OSXvnc) as an alternative to the Mac OS X Leopard built-in server. I like to use Vine Server when I need to connect to one of my Macs from a non-Mac system, or if I want to use reduced color (like when communicating over a slow internet connection, and the built-in client/server doesn't like that).
