Quite Universal Circuit Simulator (QUCS) is a great tool, and one of the best open source RF/Microwave tool I have used so far. It s easy to use simulator for DC, AC, Sparameter, transcient simulator.
It also includes harmonic balance (HB) simulation although work is still needed for this HB to work properly.
Furthermore it has the possibility to include VHDL and Verilog file circuit using the FreeHDL and Verilog Icarus engine tools respectively.
I have used it to calculate the group delay of two simple low pass filter
and I obtained great results (see below)
Note that: My group delay calculation are based on
Gd = - (D(S21)/D(w))
where w = omega in rad/s
You can load the example files at the Qucs web site example
there are examples for AC and Sparameter setup
or directly (right click save as)
Using Sparameter simulation groupdelay_sp.sch
Using AC simulation groupdelay_ac.sch
If you are an RF/Mw geek like me, have a go, you would not regret it.