This video explains how to create an AddIn for SharpDevelop 2.0:
Tutorial1.wmv (5.2 MB, 05:19 m)
It is the first part of a series that will demonstrate some example AddIns and how to create them. The next parts will be based on the AddIn we set up in here, so make sure you follow the steps!
A page in the wiki contains the full list of the tutorials published so far.
EDIT: Note that the repository URL has changed since the video has been recorded. The new address is svn://sharpdevelop.net/sharpdevelop/trunk