What have you paid a lifetime/one-time fee for that was worth it?
(Previously, but now pretty old.)
(Besides your MeFi membership, obviously.)
Many things have the option to buy a lifetime membership or a one-time license fee. (For example, the Plex Media Server software for running a home theater has the Plex Pass for added features, and you can buy a Lifetime Pass as opposed to paying monthly.)
Or REI sells a lifetime membership that's totally awesome.
I am thinking more about "what unlocked the most features or delivered the greatest value" and less about "what did the most good in the world," but I would love to hear anything.