Circuit Simulation Software
LTspice is free and works well for switching power supply designs.

I also use Paul Falstad's Circuit Sim for concepts.

When it comes down to designing power supplies (which is pretty much the only thing I use circuit simulators for) I usually rely on application notes and already designed supplies. If they use LT parts, I might try and optimise the design. WEbench by National Semiconductor is quite good, but I wish it gave you more freedom.