When designing some new gadget, like a PWM controller for a dew heater or sensor cooler, it helps to be able to reliably simulate function and performance.
A good free software tool is LTspice from Linear Technology
http://www.linear.com/designtools/software/
It is well maintained version of the Spice simulator. It comes with a library containing many common components. Most component manufacturers provide Spice models which are easy to load into the LTspice library.
There is a users group
http://groups.yahoo.com/neo/groups/LTspice/info which can provide help and has a library of circuit ideas.
Other free Spice versions exist. A few are described in the Electronics Design article
http://electronicdesign.com/analog-a...nalog-circuits
There is much information on the web to assist in learning Spice.