Wizards of the Coast has released a 5e System Reference Document… and unlike 4e, it’s released under the Open Gaming License v1.0a — the same license they released D&D 3e and 3.5.
A quick skim of the Product Identity and Open Game Content declarations in the license looks very much like that of D&D 3.x. It looks as though they’re going back to the best release model they’ve found for D&D. You can now build on it and publish, without feeling like you’re running under the radar.
I’ll be interested to see if they extend this to follow-on products. With Pathfinder, pretty much all the crunch is (or becomes) available under the OGL, but in 3e and 3.5 this was not the case, almost all the later books were almost entirely closed content.