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ShareEnviro will especially appeal to users who want to be able to customize every aspect of their desktop. This shell is open source, so it's free of charge.
WindowBlinds, by Stardock, used to be a full-blown desktop shell replacement, but now it's a tool for improving the look and feel of the Windows 7 shell.
You won't be adding widgets and new and improved menus with WindowBlinds. But the enhanced appearance of the Windows 7 interface can be fairly dramatic. This shell costs $19.95