hbfree - free RF&MW harmonic balance simulator - Home page of free RF/microwave harmonic balance circuit simulator. It is useful for analog nonlinear circuits simulation with diode and MESFET nonlinear elements.
PSPICE.com - design community in action - pspice.com - an eda product and software community site for pspice user. Includes analog and mixed analog/digital circuit simulator information from OrCAD.
Schematica - Windows software for design of active filters and linear circuit simulation
The Designer's Guide - A source of in depth information useful to users of circuit simulators such as SPICE.