As described in the June-2006 Scatterpoint, software for the 'Simple and Cheerful L Band Synthesiser' is here
Circuit Tools & Simulation
Free utility from Avago (formerly Agilent) covering Transmission lines, Transistor Parameters, cascaded noise etc. Its not ADS, but its free!
Download Link - 14MB
TXLine - Transmission Line Calculator
This free software from AWR analyses/synthesises microwave transmission lines
for microstrip, stripline, coplanar and slotline.
Download v1.1 Here, or Newer 2003 Version
RFSim99 - Microwave Circuit Simulator
This free shareware from HyDesign can perform linear S-parameter simulation, tolerance analysis, stability circles, matching, tuning and filter design.
Download v1.05 Here - 2MB
QUCS - Circuit Simulator
Quite Universal Circuit Simulator is an Open Source circuit simulator for Windows, Mac and Linux with schematic capture,
s-parameters, flexible graphing.
Free Spice-based simulator and other tools from Analog Deices (formerly Linear Technology) with full schematic entry, libraries, custom symbols etc.
Download Link ~5MB
Free tools for DDS, rf calculators etc.
Sonnet Lite is a free, limited capacity, version of a professional EM Solver for microstrip and stripline design.
A student version of the Ansys Electronics/HFSS 3D EM Solver suite - the High Frequncy Stucture Simulator
CST Student Edition
A free though restricted version of the CST Electromagnetics suite
Feko Student Edition
Feko Student Edition (with Winprop) is a free, limited capacity, version of a professional EM Solver for Antenna design.
A free but limited 3D FDTD solver - Matlab or Octave and Python are used as an easy and flexible scripting interface
Super Filter design program called "LC" or "Elsie"
This very useful program from Jim Tonne WB6BLD has a free version,
and his website has other handy utilities.
AADE Filter Design
Free lumped/crystal filter design and analysis - archived copy:
G4JNT Software Archive
Courtesy of Andy Talbot G4JNT, useful Beacon and other Software utilities.
Paul Wade W1GHZ (ex N1BWT) "On-line Microwave Antenna Book"
This large and thorough work is split into sections for easy downloading.
Link to table of contents of the "Microwave Antenna Book"