Previously we looked at the gEDA toolset for circuit design, and concluded that it needed more work before it is competitive with Eagle. It turns out that the version shipping with Ubuntu 9.10 is 1.4.3-1 which is by far inferior to the latest version 126.96.36.19991004. It's still not perfect but it is about 50% less annoying to use and I can actually see myself using it for a project now.
The spice simulation tools for gEDA are not perfect by any means but good results can be achieved. This is a good tutorial that shows how to use gschem and ngspice to perform a simulation.
There are some other interesting tools for working circuit simulation. Dataplot is data viewer for ngspice and gnucap simulations results. The developer Werner Hoch has also released a spice circuit optimizer and some other interesting tools.
If these tools keep getting better at this rate it's only a matter of time before gEDA and other open source tools are competitive with Orcad and Eagle. As time goes towards infinity open source software will converge to the winning solution.