A free chess opening database? I'm looking for a chess opening database that is reasonably complete and computer-readable. It should be available either under a liberal free license (like, say, CC-BY, no GPL), or under reasonable commercial license.

Are there any?
No.

The reason for a no, lies in your question. If you want "liberal free license" you are asking for free software and “free software” is a matter of liberty, not price, since you specifically said "(no GPL)" then your request is contradictory.

However, if you do want “free” (as in “free speech,” not as in “free beer”) opening database, there are some options.

You can build small (1 MB), medium (25 MB), or large (58 MB) opening books for Crafty, that will give you enough material for a couple of years, maybe even more.