Circuit Simulation
The first thing that I need is a circuit simulator. I've run into a surprising amount of trouble here.
Lush Projects
http://lushprojects.com/circuitjs/
Haven't tried it yet, but it looks pretty good. Based on the Java one I used to play with.
Circuit Lab
Ok, but the free version was very limited. Subscriptions are expensive $15/month (cheapest plan is fine for me).
QUCS
Desktop FOSS. I kind of liked it but it was a pita to use and after figuring out some very odd quirks, I started running into problems with it's simulation. Basic oscillator circuits would crash the simulator.
The other significant disadvantage is that there was very little support.
Spice
Tried a few variants. Extremely difficult to use, and I ran into the same issue as QUCS.