Way back in the day when Vista was known as Longhorn there was a sidebar that some of us began to love. There were, however, issues with it that prevented Microsoft from releasing the sidebar in later releases of Vista.
There is hope that we will see it again in the February CTP that should be released around the 21st of the month. This would be an exciting feature to see back in Vista especially with the enhancements that they have made.
If you never like sidebars because they take up too much valuable screen space, then there is no need for you to worry. With the Windows Vista Sidebar you will be able to prevent it from always being on top, that way you will only see it when you are viewing the desktop on your computer.
News Source: Longhorn Blogs