There are many, many competitors in this space (visualPing, changeTower, versionista, followThatPage, trackly.io). Most have similar pricing to Dffer, but some are completely free (possibly with worse UI).

I wonder whether this sort of freemium model works, because there are some people who's use case (like mine) absolutely isn't worth spending money on, but I'd like more than 3 checks for free. Obviously I'll take a worse UI that's free over spending money on such a ridiculously simple app. I could literally buy an EC2 instance just to run 10-20 daily cron jobs for less than signing up for Dffer.

My use case is to use trackly.io to see when new firmware/software updates come out for devices I own. But of course, that's never something I'd be willing to pay for.