I've been teaching private guitar lessons for the last year, and while I make a point of tailoring lesson plans to each individual student, I'm always looking for great resources with broad applications. For beginners, the Hal Leonard Guitar Method is an effective starter series. For intermediates, 101 Guitar Tips is rich with invaluable advice. For jazz players, The Real Book is a must-have.
There are plenty of other books which I own, recommend, and use for teaching as well as my own development, but it's often helpful to have a big full-page printout of a single concept I'm trying to convey. I used to make some myself until I found Teach Wombat. The owner, Rob Hylton, sent me a sample of the massive collection of guitar, bass, and general teaching materials available for sale on the site. I now use them regularly in my teaching to supplement whatever primary material a student is using.
For example, take a look at this free PDF offered on the site: First Guitar Chords. It seems simple enough; it's just the five open major chords and three open minor chords that are fundamental to every guitarist's skill set. But it's rare to see them so clearly defined, together on a single page, formatted to be as large as possible. The Hal Leonard method introduces them one by one, over the first 60 pages. That's a great way to start, but having this Wombat printout handy helps the student maintain this knowledge.
My other favorites include several types of blank staff/tab paper and chord grids, blank full neck diagrams, barre chord shapes, power chord shapes, and scale shapes. If you're a fellow guitar or bass instructor, take a look around Teach Wombat, take advantage of the handful of free PDFs like the one above, and preview all the other resources Rob has to offer.