Quite Universal Circuit Simulator
QUCS is an alternative to the various SPICE based simulators with the additional benefit of being mulit-platform by design.
It is available in the Ubuntu 12.04 repositories or a PPA for a more recent version.
Works well on Ubuntu 12.04
Lots of options
Forced file locations. QUCS project files in the ~/.qucs/. I know some people like programs to organize files for them, but I find it a pain because they are seldom organized in a that is easy to use with a revision control system (git).
Error/Warning messages are not very helpful.