Overview and demo (skip about 1st 10 minutes - it's setup for the webconference session that was recorded; just drag the slider at the bottom of the video). Ends after an hour, but then questions follow.
Also a lot of links related to diigo underneath the video on the page.
A collection of tutorials about Diigo - using Google Notebook rather than Diigo to compile the list. (It's a good illustration of the use of Google Notebook, too!)