I know everyone else has already talked about Gliffy, but I wanted to add to the discussion. I have been using it to create graphics for lessons, and I am really impressed. The most recent one was on using clues in the textbook (pictures, tables, headings, etc.) to help you prepare for the actual reading. This graphic is one of the ones I included in the lesson. It isn’t spectacular or anything, but I like it!
Check Gliffy out if you haven’t. It is really cool!