Program for simulating circuits on linux
The gEDA project supports the Icarus Verilog (digital only, though Verilog-AMS support is planned), GTK Wave, ngSpice, and Gnucap simulators.  It has the advantage that you can compose your schematics, simulations, and PCBs in the same flow, instead of having to manually copy everything from your schematic editor into the simulator.

If you want to evaluate more options, the Wikipedia comparison of EDA software, Electronic circuit simulation, and list of free (as in freedom) simulators pages all contain useful links.