Arduino is popular because it allows people such as artists who don't know much about electronics to construct electronic and electromechanical devices. However, quite a few users seem to find it difficult to produce accurate schematic drawings that are easily modified and are suitable for posting.
CadSoft's Eagle software is widely recommended, even for amateurs, though it does have something of a learning curve if you need all the functionality. In fact, it's quite easy to learn to draw schematics if you know where to find the right library components. Arduino users aren't usually interested in designing printed circuit boards.
I think it might be interesting to put a collection of commonly-used items in the Arduino library directory. The key items would be complete Arduino boards presented as symbols, but drawn to scale with the connectors in the correct places. To test the idea I've made a library for the Arduino Mega 2560 (http://www.music.chrblee.net/arduinoeagle/). The package also looks like the Mega card, but it's there only because you can't make a device without it.
Perhaps Eagle and Arduino power-users could find the time check it out before I think about taking the idea further.
Here's a drawing of the Arduino midiTranslator schematic adapted for the Mega and a MIDI-shield. The junctions in place of symbol pads give a less cluttered appearance than the standard pads and they do no harm.
I found a scale drawing of an LCD display in the Eagle library & changed it a bit to make it more generic. For present purposes you don't always need the exact library device corresponding to the component you're using; often it's only the pinouts on the symbols that matter.