Back in 2005, I uploaded a bunch of crude little pictures to my Flickr account [edit 12 years later: this account is gone] and called them my “Whiteboard Icons” set. Hundreds of people have, over the years, found them to be useful on their blogs, flowcharts, and more.
Today, I finally added a couple new icons to the set. Only took me five years.
Enjoy.
Update: Various sizes and scalable versions are available in the whiteboard_icons git repository.