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openCanvas

openCanvas is an art program for Windows which was released as freeware until version 2.0 (which lacked network support anyway). Because of it's network feature, oC 1.1 is widely used for collaborative drawing, even though it's not the most stable program you can imagine, but there aren't really any better alternatives.. yet.

Anyway, as it's Freeware, you can get it here:

openCanvas 1.1

Also, I recommend using the non-shitty oC server if you want to host, because oC itself is likely to crash, especially when more than 2 persons draw or too many layers are used.