malmal - Draw together on a nearly infinite canvas with your friends or with strangers.
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
DrawPortal - DrawPortal is a drawing app that enables users to create beautiful drawings.
draw.io - Online diagramming application
Anondraw - Draw and paint live with friends or strangers on a big canvas. There is also a gamemode and gallery.
yEd - yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.