WPBookmark is a wordpress plugin that allows users to bookmark posts/pages. you can selectively disable bookmarking feature on certain pages/posts.
go to plugins->Add New->Upload. upload zipped plugin folder and then activate plugin.
once the plugin is activate, you'll see a bookmark button on every page/post. in one click on the button creates bookmark and saves to DB but on the consecutive click by same user, the button will toggle between bookmarked and unbookmarked.