Math Open Reference is a free open source interactive textbook. I’ve been referring to it in class for years because it works marvelously with my smartboard.
I like to think of it as my own collection of Geometer’s Sketchpad type applets that can be used for every concept…without having to create it myself. In addition, every main applet can be used in “full screen” mode, which looks and feels great when projected.
I also list Math Open Reference as a site of reference for my Geometry students needing homework help. There is no greater moment than being witness to my kids experiencing that “I get it” moment and MRO has been very helpful in helping to get them there.