Most recent program: I wanted time-lapse photos of some potted plants I'm trying to grow. So I stuck QR codes on the pots and take a picture of each one each day with my phone. That goes to Google Photos, and a cron job scans new photos, recognizes the QR code, and copies the photos into directories for each plant with filenames that sort chronologically. Most of the plants died. I am a better programmer than gardener.
Most used program: I keep a journal of practical daily happenings in Emacs org-mode, organized hierarchically by year/month/day. It was too much hassle to add new entries (I'm lazy), so now I have a utility that inserts journal entries in the correct place from the command line, e.g., "add-journal 'talked with roofers to get estimate'". It's turned into an ad-hoc database for recording things like my weight, also, which gets parsed and turned into a graph.