Much has been written about creating 'widgets' or sidebar panels in Umbraco.
In a recent project, we decided to implement our on-page widgets as 'pages' in the Umbraco content tree. This has a number of advantages. Users can be provided with a number of different templates and edit the content with familiar tools. It also means that we can create a central 'pool' of widgets which can then be added to pages using a content picker.