Spice

Spice is a circuit simulator for the Unix platform to analyze and design IC boards without having to build and design those circuits. This piece of software has been around for a while and I totally forgot that it was freely available for not only education but actually as design utility.

There are others out there that are commercial and have more functionality and such, but overall, you can't beat the free from U.C. Berkeley. I remember spending many late night hours in the lab with this piece of software and if you're looking for something to help do IC design, don't forget some of your old college software friends.