Leopard's screen sharing uses the standard VNC protocol, and so it is possible to use it to share screens with other non-Leopard systems. I run the VINE Server on my older 10.4 Mac, and it appears in my Finder under Shared, allowing me to click the Share Screen button to bring up its screen.
This may only work with the VINE Server because it supports Bonjour. It would be interesting to know if there's a way to get Windows PCs running a VNC server to show up.

