From the codex:
Add the "Press It!" bookmarklet provided by WordPress to your browser and you have a shortcut to create an article with a link to the page currently displayed on your browser!
If you don't like a bookmarklet, use our friendly broswer sidebar, which can be used in a similar fashion.
As far as I can determine, this is the only mention of a "sidebar" alternative to bookmarklets in the codex. Can someone shed some light on this feature? How do you invoke the sidebar?