SPICE Simulator at Linux
I'd say either ngspice with gspiceui (part of gEDA I believe) or LTSpice with wine as Renan has already mentioned.
Here's a screenshot of ngspice on KDE (with one of the graphical addon packages like nutmeg):



I have a linux box and I use the second option (rarely, since I'm mostly on my Windows laptop), simply because I'm used to LTSpice.  

There are also many others, including some interesting variants like eispice, which claims to be the only open source SPICE that provides native IBIS model support (this may be pretty useful for many high speed digital designers on a limited budget - I will certainly be checking it out). It was aimed at PCB signal integrity simulation initially, but has expanded to include more general purpose features.