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.