Is there any EM simulation tool in Linux?

Is there any EM simulation tool in Linux?
I know CST has a Linux edition but it can not build a model.
Is there any EM simulation tool which not only can do the simulation ,but also can build a model.

Empire XCcell (FDTD based) is available for Windows and Linux.
http://www.empire.de/

For 3D planar simulation model (PCB/RFIC), you can use Sonnet Professional, which is available for Windows and Linux.
http://muehlhaus.com/products/sonnet-planar-3d

For 3D structures, HFSS is available for linux and supports up to RedHat5 for both solving and GUI (and LSF support)

For planar structures, ANSYS' Designer with HFSS Solver on Demand is available for Linux and can call either the 2.5D MoM solver (PlanarEM) or HFSS to solve the same setup.

Have Fun

Also EMPro (agilent) for 3D structure, for planar - ADS.