http://cnx.org/
In addition to full books Connexions has modules such as the physics of music or how a police radar works.
For example, accelerated 1D motion can be found here: http://cnx.org/content/m13834/latest/
It features clearly written material with plenty of diagrams.
Overall the site appears to be a gold mine for any teacher interested in flipping. Remember that flipped materials don't have to be videos, they can be readings as well.
