malmal - Draw together on a nearly infinite canvas with your friends or with strangers.
LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.
DrawPile - Drawpile is a drawing program that lets you share the canvas with other users in real time.
draw.io - Online diagramming application
Aggie.io - Draw a picture together with your friends in real-time over the internet in your browser.
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.