I see lots of people on AAC use LTspice.

Is this free to download. I bought Proteus, I imagine I hardly scratch the surface of it, but like the idea of being able to simulate circuits.

 

Yes, it's free from Linear Technology. Additional libraries of spice models are freely available from the Yahoo LTS User Group.

 

Hello,

LTSpice is made by linear technology:
http://www.linear.com/designtools/software/

There is also Tina made by Texas Instruments:
http://www.ti.com/tool/tina-ti

The EDUCYPEDIA has links to some other simulation programs:
http://educypedia.karadimov.info/electronics/easoftsim.htm

Bertus