The Dock may be moved to any edge of the screen, and may be pinned at either end. This feature has no GUI interface and was actually disabled earlier in the development cycle, but has been re-enabled in 10.0. It can be activated by a simple and widely documented hack. The result is a pop-up menu for pinning and relocating the Dock (shown above).
Take a look at the screenshot. Sure, the options can be accessed via terminal commands (except the dock on top - that's gone in Leopard) but you know, it would be nicer if Apple let normal non-Terminal users get more customizability out of the OS. I know a lot of users who would love this extra push of customizability. I mean, I've done it before but I wish I had options in System Preferences or in the Dock menu for this.