Drawpile is a Free/Libre networked drawing program that allows multiple people to sketch on the same image simultaneously. It supports the OpenRaster image file format and thus works well with applications such as MyPaint, Krita and GIMP.
You can paint with a pixel pen, soft brush or a watercolor brush. Brushes can be organized into presets and quick access tabs. Use a dedicated eraser tool or turn any brush into an ad hoc eraser.
Both brushes and layers support various color blending modes.
Host drawing sessions locally with the built-in server or using a dedicated server. Discover public sessions with the list server or join a friend with a handy private room code.
Drawpile provides a wide variety of tools to help manage sessions:
Record a whole drawing session using Drawpile's recording feature. The recording can later be played back and exported into video, or used as a backup. Drawpile also has basic support for creating short animations, using layers as frames. Animation specific features like onionskin layer mode and a flipbook preview are supported.
You can follow Drawpile's development over at GitHub. Help is always appreciated, but you don't need to be a programmer to participate! Ways to help include:
If you know C++, help with application development is very welcome, especially things related to Windows and Mac compatiblity.
If you're a web developer interested in helping out, the source code for this website is up on Github! Another project (which I haven't yet started), that I could use help with is rewriting the old server web admin site for version 2.0.