By default Mountain Lion displays the All My Files item in the Sidebar, which allows you to see and/or group all of the files on your Mac. Many people find this folder handy; but many others do not, and end up removing it from the Sidebar.
If you by chance removed it and decide later that you want it back, you can do so from Finder preferences. Click Finder on the menu, click Preferences and then check box box next to All My Files under the Favorites category. You can show or hide other Finder items from this window as well.