Dock Spaces is an application for the Mac that allows you to have multiple (up to 5) different Docks. Despite the name, the Dock does not change when you move between Spaces. Rather there is a control added to the menu bar that allows you to select between different Docks. If you are using Spaces to keep your applications grouped, this is still a nice application to have - even though it's not completely automatic, it's still nice. Oh yeah, it's also free and it only works in Leopard.