The first to be posted was for the complex Arduino Due (above) by user Graynomad. Then user Pighixxx used similar layout to do diagrams for the Uno, Mega, Esplora and more.
For readers, I link to the forum posts for various diagrams. This allows you to get the latest version in case of revisions (the forum is keen on spotting any problems which keeps the quality of the diagrams very high).
- Arduino Uno with additional diagrams on the ATmega 328 and ATTiny chips
- Arduino Mega
- Arduino Leonardo with Mega 32U4 chip diagram
- Arduino Due
- Arduino Esplora
- Arduino Nano
- Arduino Mini
Also with the Uno, Pighixxx laminated a printout and mounted it on a plate and mounted a real Uno which makes a great prototyping plate (probably more detailed than a beginner might want, I would say a great target would be for technical or college level classes).
One doesn't need this level of detail for their first Arduino project but for midlevel to advanced work, these are wonderful and bring together information it could take hours to research. Thus you have more time for actual experimenting (and debugging).