Cadsoft Eagle (PCB Design)
Tutorials on designing schematics and PCBs with Cadsoft's Eagle PCB design software
Alright, it may not be the prettiest software on the web and like anything it has its quirks, but for better or for worse Cadsoft's Eagle is pretty much the defacto standard in open-hardware, and we've gotten used to working with it ourselves so why mess with a (mostly) good thing? While there are other competing 'genuinely' free PCB design packages out there (KiCad, for example), the commercial but freely available Eagle has one big advantage ... it's a mature product that has a lot of small refinements, and once you get used to working with it it's relatively painless and productive. As a reasonably mature product, there are also a number of tutorials and forums out there for it, and some decent free Eagle libraries (ahem ... hint hint, nudge nudge) that might come in handy if you have a problem or pressing project.
While Eagle is a commercial product (we don't expect that to change despite the fact this it was recently acquired by Farnell), there is a free version available that allows you to produce boards up to 80x100mm using 2 layers. In reality, you can do a lot with a board that size, and if you really need more space than that or need 4+ layers you're probably working on a project that justifies the expense of buying a commercial license, or purchasing a non-commercial standard license to allow multi-layer and larger boards.