JollysFastVNC is a shareware VNC client software developed by programmer Patrick "Jolly" Stein (http://www.jinx.de/JollysFastVNC.html, $39 suggested after trying it out). It's a little quirky, but in my testing, it's been full-featured and reliable. The software fee doesn't include technical support.
The program seems to be able to connect to Macs using Screen Sharing in Leopard and Snow Leopard without VNC compatibility enabled, as well as to any VNC server.
Launching the program shows a Server List, which displays any VNC servers and Macs with Screen Sharing enabled that are available via Bonjour over a local network or through Back to My Mac, as well as any remote servers you manually add (Figure 10-2).
Figure 10-2. The ...