I'm sure there are plenty of schematic tools available. Which ones do you use/prefer?
Feel free to also list tools which aren't completely free but very popular.
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I use gEDA/PCB. The file formats are open and ASCII. The open file formats make a wide variety of EDA automation tasks possible. The ASCII format makes them easy.
I have switched from Eagle to gEDA/PCB. I have found gEDA to be a more productive tool. The schematic capture is better but the PCB layout seems more difficult. The scriptability is what has made the difference. There are also tools for simulation.
Be careful of choosing a free version of an EDA tool that is crippled or a tool that locks you in to a specific PCB vendor. There is a learning curve associated with any EDA tool or other complex piece of software. It will be very time consuming to switch tools.
The footprint library that I use is available at http://www.luciani.org Also I have a variety of EDA automation scripts on my site.
As an example of gEDA/PCB I did a remix of the Drawdio circuit design that ladyada did (CC 3.0 BY-SA). The remix includes the EDA files and documentation. All of the files are at http://wiblocks.luciani.org/remix/index.html
A couple of additions --
XCircuit http://opencircuitdesign.com/xcircuit is an open source package that produces some very nice looking schematics. The most extensive example would be the open textbook at http://www.ibiblio.org/kuphaldt/electricCircuits
If you use La/TeX you can create inline schematics using macros. This could be useful for very simple schematics but I can see this getting very cumbersome very quickly.
The de facto standard (at least acc. to my impressions) is Eagle CAD. It is not open source, but there is a free non-commercial license availlaible. It is limited in PCB production (2 Layers, 10x16cm) and can handle only one schematic sheet.
I like it a lot.
A good open source alternative is Kicad. But I have not used it often enough to give good advice.
You haven't said what your schematic is for. If it's purely an illustrative schematic-- one that won't be used to generate a netlist (and/or PCB) --then you may find that general-purpose drawing tools work very well, often much better than engineering applications.
For example, we often publish circuit diagrams on our blog, and write up projects that we contribute to books and magazines as well. In situations like this, I use Inkscape to make great looking schematic diagrams that show exactly what I want to show. (You can see a few examples in this article.)
If you need to use a dedicated schematic package, as part of a PCB layout, then I'd also recommend gschem, part of the gEDA suite. It produces very clean vector-pdf output that make for good looking printouts. It has the limitations that other EDA tools have-- a learning curve and less control over the visual output than an illustration package like Inkscape.
I don't use it (I have a very good PCB CAD package), but TinyCAD is very popular. I tried it once, and it was quite easy to use.
In my eyes the best software is Cadsoft EAGLE. Its the best schematic editor on the market. The new version V6 is easy to learn and easy to use. We also use it in university.