Kent Sutherland, the developer of the well-known Chax, a utility for making iChat easier to use, has a nice new utility for improving Leopard's Spaces virtual desktops as well. Warp is a preference pane allows you to switch between Spaces just by hovering the mouse cursor on the screen edge. You can set the switch delay as well as require a modifier key if you prefer. In some ways this is such a natural addition that it's disappointing that it wasn't built into Spaces from the beginning.
Warp is a free download, but donations are requested.