There are decent lists of other freeware RF simulation utilities here and here if you’re interested. Having said that, RFSim99 is handy if you want to design a basic filter, impedance match, resistive attenuator, or create/view 1 or 2 port S-parameter files.
I should make it clear that it performs basic RF simulation only, and is nothing like Genesys/ADS, Microwave Office, Ansoft Designer, HFSS, or of that ilk, so if you want a full-on EM solver, this is not your program. RFSim99 is a freeware RF simulation software utility. I’ve been using RF sim 99 (aka RFSim99) for over a decade, and it’s proved invaluable again and again during that time, and continues to do so.