...under perpetual construction.
fasthenry is a three-dimensional inductance extraction program, which can compute the frequency dependent self and mutual inductances and resistances between conductors of complex shape. It is available in C source code form at the Research Laboratory of Electronics at MIT, Computational Prototyping Group research codes page.
To quickly verify variometers and coils designs, I have written a a simple C program (coilgen) that generates fasthenry input files for arbitrary coils systems; with a simple description of the main physical dimensions you can generate a complete fasthenry input file to compute the self inductance of a coil, with no restrictions on length, diameter, turn spacing, etc. or even compute the minimum and maximum inductance of a variometer.
Here you can find a brief description of how to set up a simulation with fasthenry and coilgen.
For Windows users, I have compiled the sources using the Code::Blocks IDE, which uses the gcc compiler provided by MinGW; the resulting executable and the sources are available in this coilgen for Windows package.
A simple fasthenry executable for Windows is available on the Whiteley Research Free Software Tools page.
A list of electromagnetic simulation codes available on the internet can be found on the Missouri University of Science and Technology (formerly University of Missouri-Rolla) page.