Comparison
66music and JustinGuitar
Short version: if you are starting the guitar, use JustinGuitar. It is the best free structured guitar course there is, and this page will not pretend otherwise.
What we are instead
We are not a course. There are no video lessons here and nobody teaching you where to put your fingers for the first time. 66music is the drill material for after the course: 264 recorded guitar licks that climb from 1 to 66 in four styles, every one hearable, loopable and slowable without changing pitch — plus the same shape on piano, bass and drums, 1,254 exercises in all.
Use both
Learn a concept with Justin, then come here and drill until it sits. The two do different jobs, and doing both is what a practice week actually looks like.