Perhaps everyone knows about this already but I have found the free potrace program very useful for tracing bitmap line-art into svg format. The resulting svg file can then be imported into Inkscape for further manipulation. I have also produced diagrams using various LaTeX drawing macro packages in eps format, converted them to pbm format using Ghostscript and then to svg using potrace. This could be a way of converting suitable public domain clipart from bitmap format into svg format for use with Inkscape. Thanks Bill Peel