Doug LaBrecque's Multi-phase Technolgies web page - contains a lot of ERT and many ERT links
MPT ERT
Alex Kaminsky's ZOND geophysical inversion software. Includes 1D, 2D and 3D resistivity and IP codes.
Zond Geophysical Software
The 2D code Zond-Res2d contains a manual in English and a
movie to demo the software.