This online drawing tool can be downloaded and used as an app on your desktop. Plenty of sample charts and diagrams get you started. Easy and intuitive to use.
If only there were a free version of Illustrator … or FreeHand … or CorelDraw. Inkscape aspires to be all these things and has lots of support.
A pretty good alternative to Illustrator that is free for individuals.
Vector drawing program promises intuitive tools, real-time collaboration and scale-able graphics.