If I understand your question, you want to add something to your sidebar that is not necessarily a standard function or feature of WordPress. But, any/all modules you add to your sites sidebar and/or footer are, in fact, widgets.

In nearly all cases, you’ll find that the Text Widget is your best friend. It allows you to not only add text blocks to your sidebar, but you can also paste HTML code into a text widget, which allows you to make a text widget serve just about any custom purpose.

Feel free to re-post your question in the Q&A Board if you’d like to clarify and go further. This will help other folks benefit from your question.

Hope this helps,
Shawn