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There are tons of software packages out there for simulating eletronics circuits. It really depends on what you want in the way of features and what typesw of circuits you want to simulate.
For analog circuits an example would be LTspice from Linear Technology (http://www.linear.com/designtools/software/#LTspice).
For digital circuits you could go with KTechlab (http://sourceforge.net/projects/ktechlab/) as long as you don't mind using software that is in beta.
For mixed signal simulators there is NL5 (http://nl5.sidelinesoft.com/) which is free as long as you are a student, which I assume you are.
Those are just a few examples there are loads out there if you just do a Google search.
PSpice is the preferred sim software in most EE programs. http://www.electronics-lab.com/downloads/schematic/013/
@eSpeX Is PSpice free? I was pointing out some free alternatives since that's usually what students are looking for unless their course requires a specific software. PSice looks good though. I like the features.