I tinkered once with hash of a user's passphrase, salted, and then using 4 digits from the middle of the resulting hash as a username. I then used a different 4 digits as a cookie token so they only had to enter it once per sesssion. It's not terribly secure, it's not collision-proof, etc, but you may find some use from the basic principle - it's quick and easy and was useful on the scale I needed it for :)

Edit: Point of note: The same people who wouldn't type 16 characters of username and password would happily type 30 or 40 character passphrases... There's some novelty to it, apparently :)

I gather that UA info is surprisingly close to unique - mix in installed fonts and I think you're there.

What's the context? When they use your website?

What do you mean by "identifying a user"? Simply knowing that they're the same person who performed a specific action earlier?

I edited to clarify: I should have specified "Anonymous in the way that 4chan users are anonymous to each other."

And yes, as in "same person who performed a specific action earlier".