New answers tagged diagram
A great online solution, one that integrates well with Google Drive is Lucid Chart. There are paid plans, but it's mostly free. I've created tonnes of diagrams for it and this is a typical implementation.
yEd is good at this, is free and has Windows, Mac and Linux versions: http://www.yworks.com/en/products_yed_about.html
Top 50 recent answers are included