I pretty much use YouTube as a MTV replacement to watch music videos only, so won't speak to the quality of instructional material you find there, but I was still in the military as recently as 12 years ago and the training material in the military was still terrific. This includes unclassified material, though I don't know how you'd find it as a civilian if, for whatever reason, you were interested in learning how to perform maintenance on a modern humvee or what not.

The technical manuals are all available through Army epubs, but even though they're not classified, you still need a common access card to login and use the site. A lot of them get mirrored somewhere the public can find them, but I would definitely not call the sprawl of where you might find them very searchable.