Open Source Drawing Tools

Inkscape
http://inkscape.org/

GIMP (just released version 2.8! :)
http://gimp.org

Krita
http://krita.org

Paint.net
http://getpaint.net

Cheers!
--Brylie Oxley
http://brylie.gnumedia.org

GNU.media
http://gnumedia.org

Libre Geoscience
http://libregeo.org

Dia
http://projects.gnome.org/dia/

And for flowcharts with 100% more ascii, there's always
http://www.asciiflow.com/

MyPaint (thanks Petri!)
http://mypaint.intilinux.com/

(Via p2pu-dev mailing list - great list, a must share!)