RFSim99 is an old but still very useful piece of free software for designing and simulating RF circuits such as filters, attenuators, matching circuits, RF components with S-parameters and so on. It is not a high end tool, but is more than good enough for hobbyist and ham level designs.
One issue up until recently with RFSim99 was that it was so old that it would only run on Windows XP computers. To run on a modern PC you needed to use a virtual machine, or the Windows built in XP emulation, which was only available on Professional/Ultimate Windows versions and has since been discontinued in Windows 10.
However, we have just found AD5GG's blog where earlier in the year he posted a standalone version that actually works in modern Windows versions (7, 8, 10) without the need for any virtual machines.
RFSim99 is fairly simple to learn. Just place down two measurement ports (in/out) and lay down your RF components on the grid. Then connect them up with a wire and place grounds. Click on the simulate button to see a graph of the response and return loss. There are even built in calculators in the Tools menu which can automatically design and simulate filters for you.