Schematica - http://www.schematica.com Windows software for design of active filters and linear circuit simulation
Hbfree - http://hbfree.sourceforge.net/ Free command line RF/microwave harmonic balance circuit simulator, for analog nonlinear circuits simulation with diode and MESFET nonlinear elements.
Designer's Guide Consulting - http://www.designers-guide.com Provides verification and simulation services to companies involved in integrated circuit design. Includes information resource about modeling, theory and analysis, as well as a discussion forum.
Infolytica - http://www.infolytica.com Offer electromagnetics design software for both low and high frequency applications. Products include MagNet, ElecNet and FullWave.
AC Microwave GmbH - http://www.linmic.com/ Offers the LINMIC+/N microwave and RF IC design suite; mostly electromagnetics-based.
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