What free tools are there to simulate RF circuits?
For normal lumped circuits, your favorite version of spice will work fine.
For analyzing matching networks, I've used gsmc.
For analyzing a layout and extracting parasitics and computing fields, I dunno. It's usually easier to just build the circuit and see what happens.
Be sure to check out QucsStudio. I've just started with it, and it seems too good to be true. These guys put more work into their documentation than most freeware has in the code. Very nice!!
Also, this site has a free Smith Chart, RF Filter, and Opamp filter design programs that may be useful to you.
HP AppCAD, available from: http://www.avagotech.com, may be helpful
There's some possibility that 5Spice will do what you're looking for, although I can't guarantee it; you'll just have to give it a try and see.
So far Qucs Studio os very good. Translating the documentation (almost directly out of several microwave engineering books) into simulations can be challenging. SO.... do three of the tutorials FIRST to cut your teeth. I started with the L band amplifier tutorial - good stuff, but advanced, get out your text books. I'm two days in and I got a Bluetooth wave prop simulation to match measured (Nordic tools) - beat that.
You can sign up for CENOS 3D Antenna Design Simulation software Beta version for free here: https://www.cenos-platform.com/antenna-design
Disclosure: I am Head of Marketing at CENOS.