It was on this day in 1991 that Pretty Good Privacy was uploaded to the Internet [... then...] a number of volunteer engineers came forward and we made many improvements. In September 1992 we released PGP 2.0 in ten foreign languages
Fun fact: "We made many improvements" is doing an interesting lift in this paragraph, since PGP 1.0 shipped with a cipher of Zimmerman's own design, Bass-O-Matic, which lived up to its name in part by being demolished by Eli Biham over a lunch. PGP 2.0 introduced IDEA, and, I believe, the first (for the era) cryptographically credible version of PGP; a more fitting anniversary to celebrate, perhaps.
(We've learned quite about about how to engineer cryptography in the ensuing 29 years, and PGP hasn't kept up --- can't, really; so on the 30th anniversary of Bass-Free PGP, we might fittingly celebrate by finally giving PGP a well-earned retirement.)
What do you recommend as a replacement for PGP? (I'm looking for stand-alone software I can use to encrypt files on storage media, not an encrypted e-mail service.)
The high bit of the right answer to this question is that you don't want to replace PGP; one of the things we've learned in 29 years is that you don't want a single tool to do lots of different cryptographic things, because different applications have different cryptographic needs.
For package signing: use something in the signify/minisign family.
To encrypt a network transport, use WireGuard.
To protect a web transaction on the wire, TLS 1.3.
For transferring files: use Magic Wormhole.
For backups: use something like Tarsnap or restic.
For messaging: use something that does Signal Protocol.
To protect files at rest, use encrypted DMGs (or your OS's equivalent, like encrypted loop mounts).
To encrypt individual files --- a niche ask --- use Filippo's ungooglable "age".