One that sends me a mail when it’s the birthday/anniversary of someone I care about. Helped me to keep many friendships alive over the years.

Probably the only digital thing besides my self-hosted mail account that survived the last 20 years unchanged.

Not sure if that counts, but I use some scheduler libs that use cron syntax. Mostly stuff that sends threatening mails to users if the database says they are overdue with something or just some regular database jobs.