Pencil2D is a cross-platform animation/drawing application.
Pencil2D is an animation/drawing application for OSX, Windows and Linux. It lets you create traditional hand-drawn animations (cartoons) using both bitmap and vector graphics.
TundraDraw is a cross-platform ANSI drawing program written with C++, using the Qt Toolkit. Thus, it runs natively on both X environments and Windows. It features network connectivity to allow artists to draw together over the Internet.
*** THIS PROJECT IS DEAD ***
But try http://drawpile.net/ instead! It's also open source and has made a 1.0 release.
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CjopazeDraw is a real-time Internet collaborative drawing tool in the vein of openCanvas, pchat, or flockdraw, but designed with full cross-platform thick-client graphics tablet support, fast synchronising, and per-user undo and redo.