Nearly every website owner I've worked with is obsessed with users getting 404s from old incoming links. First I wrote a cron to summarize apache logs and email a report, but that involved me building redirects or a cluster of cms changes. Its evolved into a super simple tool to help site owners see user 404s in near real time and setup their own redirects until our team gets around to fixing them. Super simple from a tech pov, but it's completely removed a huge set of annoying tasks from my plate.

Toying around with productizing it as https://www.404fyi.com