<a href="https://captio.co/" rel="nofollow noreferrer">https://captio.co/</a> (<a href="https://twitter.com/EmailYourself" rel="nofollow noreferrer">https://twitter.com/EmailYourself</a>) to send quick reminders to myself. It hasn't been updated in a while, e.g. no dark mode, so there might be chance for somebody else to improve. I'd value it at $some USD/month but I might in the minority here. It was a single purchase when I installed it.
My own: <a href="https://sunclock.net" rel="nofollow noreferrer">https://sunclock.net</a><p>Shows sunrise, sunset, and twilight times, and the phase of the moon. For me it connects clock time to the physical world.