A fully open-source EM simulation software is definitely the holy grail of all designers. I wrote a small and somewhat useful bridge between the well known circuit CAD (a.k.a layout software) and the octave interface of OpenEMS. If you are not familiar with OpenEMS, it is an open-source FDTD simulator, written by Thorsten Liebig. I will keep developing this interface. The results are partially verified, making this a somewhat reliable tool. Feel free to read about some of the process here.
In general, this tool still misses a lot of the critical things you would expect from a good CAD software. Parametric builds, optimization tools, hell… Even proper waveguide ports are hardly available. But it is what you can get for free and a pence.
I am not a fan of time-domain solvers for most use-cases, but finite-element solvers are very hard to formulate and build.
