Nothing in Snow Leopard directly compares with Windows' libraries. The closest OS X feature is saved searches (known as Smart Folders), but a saved search pulls together files based on search criteria, not location. You can’t, for example, create a smart folder that contains all of the photos from two particular folders. On the other hand, you can't combine Windows 7 libraries with saved search results.
Windows 7, of course, has a smart folder feature as well.