Do you know any program that works on linux and can simulate complex circuits?
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.
There are several SPICE implementations for Linux, such as SPICE OPUS (it's free). I used the Windows version some years ago, and found it quite good. It can use Eagle schematics, which is a plus.
Try this DC/AC Virtual Lab, it works in browser, you will need Flash Player though ..
Real-time java-script browser application:
LTSpice is actually Windows software but runs well under Wine. I use it on Ubuntu.