1 - http://www.appwave.com/products/txline.html
Transmission line calculator (calculate w and lg for most planar lines)
2 - http://www.nuhertz.com/filter/
Download Filter Solutions, Filter Light, and Filter Free
20-Day Trial Period for Filter Solutions
10-Day Trial Period for Filter Light
Free version of Filter Free!
3 - http://www.aade.com/download.htm
AADE filter design and analysis v1.9.2
4 - http://www-users.cs.york.ac.uk/~fisher/lcfilter/
An interactive design package for designing analogue filters made of inductors and capacitors (Ls and Cs)
Ansoft Designer SV contains a complete high-frequency linear circuit simulator, schematic and layout design entry, powerful design utilities, and post-processing, all integrated in an easy-to-use environment. The software also includes a complete set of linear distributed transmission line models, discontinuities, vendor component parts, and ideal circuit elements.
Sonnet Lite is a free feature-limited version of Sonnet's professional Sonnet Suite, which provides EM analysis to hundreds of companies across the globe. Many major manufacturers of high-frequency components and boards depend on Sonnet to analyze their predominantly planar high-frequency designs from 1 MHz through several THz.
Maxwell SV, a full-featured subset of Ansoft's commercially distributed Maxwell 2D, is powerful, accurate software for two-dimensional, electromagnetic, and electrostatic-field simulation. It is intended for electrical engineering students studying topics relating to the design of electromagnetic components, such as sensors, actuators, motors, and transformers. Ansoft offers this version of Maxwell to foster and encourage the development of engineering skills throughout the electrical design community.