OpenEMS has a lot of mindshare in that area. Not sure how effective it is at replacing proprietary packages like HFSS, though.

Best one I know of as a direct replacement is AWS PALACE: https://awslabs.github.io/palace/stable/

Of course, you have to interact with the solver through JSON files and provide your own initial mesh. They recently added adaptive refinement but I don't know how well it works. Given what Ansys charges for an HFSS seat though can't complain!

Relatedly, is there a good open replacement for ADS/Microwave Office? I've seen QUCS/QUCS studio but don't know if there's others out there? Last I looked, the more general SPICE solvers don't support S-param blocks and transmission lines, which are pretty critical.

Depends what you're doing and how maths-y you are since you need to tell it the equations you want to solve, but FEniCs is a very good general purpose FEM code, and you can couple it to Bempp for BEM stuff.