"The free circuit analysis Spice program LTSpice, also called SwitcherCad III, is taking the ham world by storm, and I'm enjoying it myself. Here's a link to Linear Technology's software page:

http://www.linear.com/company/software.jsp

LTSpice isn't too difficult to use, and it has a good Help function. But any sophisticated program presents a bit of a hurdle. I started out wanting to write down a few commands and things I'd learned for my own reference. But the paper grew to include two step-by-step tutorials and a bit of other information for the absolute beginner, kind of an "LTSpice For Dummies". If you decide to try LTSpice, you might find this paper useful. Here are links for a PDF version and the same paper as a Word file. (Updated 10/4/2006) And here are the LTSpice files for the tutorials, in case you want to compare them to your attempt.

Tutorial #1 Transistor Amp and Tutorial #2 Lowpass Filter"