Powerline voltage, phoneline activity, clerical activity (number of people a bank teller services, for example), your car's emissions system (the equipment in the garage that tests it for inspection logs the results, so they can't be faked), the checkout at your supermarket, or at your public library - almost anything you can think of. The physical interface to the data is different in each case, of course. It's not always a device labeled "data logger".
Data logger
