FEMM looks good. I am checking into it. CAE linux looks very nice as well. I am trying to set up a virtual box to run a Ubuntu install for the software.
Try a dual boot its more effective and i know for sure CAE linux 2011 is quite stable because i am using it except for the screen flickering which can be changed by making a few changes to your grub... its your wish anyway..
A few years I have used getdp, which you can find on the site http://www.geuz.org/getdp/ . I used it for 2d and 3d calculations. It takes a little effort to learn, but it works really well and is free AND available for Windows/Mac/Linux.
This seems like an old discussion but probably worth mentionning that now sparselizard www.sparselizard.org is available open source, and will be amazing at doing what you want in 2D and 3D and much more (there is even an online example to do exactly what you want). Extremely user friendly, lots of examples online, robust, fast, documented. The force will be computed using a single liner.