A combination of Reed-Solomon forward-error-correction codes, and a binary-to-text encoding that allows common human-input errors to be detected and possibly corrected.

This results in a highly resilient code which is very likely to work.

Use for recovery codes, coupons, user account IDs, document tracking, or anywhere else you really want the user to be successful even if they've written it down incorrectly or made a mistake in typing.

It's 4 bits per character, so the same density as hex, plus RS codes.