The crystal ladder filter design program to design your own crystal bandpass filters. Version 2.0.5.2 by Horst Steder, DJ6EV, November 2015. Free for personal use.

This is like the Swiss Army knife of all simulators ! Intuitive handling with obviously unlimited capabilities ! We love it from the first day it established itself on our harddisk :-)

Linear circuit simulator featuring schematic capture, S parameter files, stability circles, auto match, filter designer, attenuator designer and RF Calculator as well as a spur searching tool. Intended for Windows XP, later versions need a workaround.

AD5GG shortly presented "his" version for Windows 10™. Read about bugs and how to circumvent them on his website. In case the website deems unavailable, try our backup. We tried it on ou Windows 10 machine and must say : GOOD JOB, AD5GG !!!

AppCAD is an easy-to-use program that provides you with a unique suite of RF design tools and computerised Application Notes to make your wireless design job faster and easier. Intended for Windows.

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AppCAD version 4.0 was released in July 2012 (needs registration)
There are several new features, mostly to s-parameters.
Download it from here : Avago Technologies
Thank you, 9W2LC, for this information !

The name says it all. Large manuafcturer Library. Intended for Windows.

Notepad++ is a free source code editor and Notepad replacement that supports several languages. Some features are : Syntax Highlighting and Syntax Folding, Multi-Document (Tab interface), Zoom in and zoom out and many more. A truly 'must-have'.