There are a couple of heavy-duty packages and a lightweight program for Linux.
The serious packages are GEDA and KiCAD. They are each a collection of programs that work well together (like Orcad); they include a schematic capture, a simulator, a waveform viewer, and a PCB layout tool. They are very sufficient except my professor requires the ".out" file generated by pspice, so I still have to use that.
The lightweight program is Oregano. It's great for quick simulations. The libraries are quick and easy to use and find parts from. The schematic capture is much easier to use and prettier than the other programs. It uses either gnucap or ngspice for the simulations, so they're pretty good. One major drawback that I have found is that the waveform viewer does not provide a logarithmic view and there's no way to get data out of it.