JustinGuitarSongs
Justin Sandercoe is the best online guitar teacher. Period. There are a lot of guitar tutorial videos out there but no one has put so much of thought and effort to create videos as well as organize them into well-designed courses categorized by levels of difficulty, style and techniques. He has 2 YouTube channels having hundreds of guitar tutorials. But for the courses you will have to visit his website. These courses have material for beginners, intermediate as well as expert guitarists. His website also has material on guitar theory, ear training, transcribing and whole lot of other good stuff. And he gives away all this for free.
JustinGuitar
This is the other YouTube channel by Justin Sandercoe.
LickNRiff
If you are more into fingerstyle guitar then LickNRiff is the best YouTube channel for you in my opinion. This guy makes learning fingerstyling look easy and fun. Videos also include various improvisation techniques and methods. Tabs for all song tutorials are available on his website. He also takes requests to make fingerstyle arrangement for songs through his website.
GuitarJamz
Marty Shwartz is another great online guitar teacher, especially for beginners. The channel has tutorials from few other teachers as well and has a large collection of videos.