Good Open source alternatives to SeisImager & RES2DINV?
Actually, there are plenty of alternative DC Inversion software;

I hate using res2dinv; unfortunately it's relatively easy to get into. I would have much preferred ZondRes2D for it's user experience.

Look up DC2DInvRes (Resistivity.net), and BERT by Thomas Gunther. You may need to contact Thomas for Bert codes, however it's all done in Python and easily read. 

Here is another one, but in matlab. (http://www.degruyter.com/view/j/acgeo.2015.64.issue-2/acgeo-2015-0071/acgeo-2015-0071.xml)

Here in another, again in matlab.
https://github.com/cageo/Karaoulis-2013