Qucs (Quite Universal Circuit Simulator) provides circuit simulation
power similar to Pspice. (That's based on my rather limited
experience with circuit simulators. Qucs is based on Qt. But I have
no background in that simulator.) It's certainly powerful enough for
engineering applications I've engaged in at various companies.
Students, "evil geniuses," and amateur radio operators will find the
getting started instructions and GUI easy to learn. On my machine it
does run slow when oscillations grow large enough to push the
simulated circuit into non-linear operation—but any simulator would
run slow on this beast. Qucs has documentation and a large collection
of circuit examples at

http://qucs.sourceforge.net/docs.html

http://qucs.sourceforge.net/examples.html#example