Wizards of the Coast, in a move which surprised everbody, has announced that it will give away the core D&D mechanics to the community via a Creative Commons license.
This won't include 'quintessentially D&D" stuff like owlbears and magic missile, but it wil include the 'core D&D mechanics'.
So what does it include? It's important to note that it's only a fraction of what's currently available as Open Gaming Content under the existing Open Gaming License, so while it's termed as a 'give-away' it's actually a reduction. It doesn't include classes, spells, or magic items. It does include the combat rules, ability scores, and the core mechanic.