Rheingold Heavy is Dan's company for educational boards. The one he started with was the I2C and SPI education board (its fulfilled kickstarter page). He brought us the the Graphic Equalizer Kit and Bubble Display Experimentation Pack.
Dan's Arduino from Scratch blog series looks at the Arduino hardware in great detail.
Contextual Electronics course for learning to build boards
Chris wrote about his Photon based garage door opener on the Linker blog
TinEye for searching schematic snippets