There's also https://github.com/thp/urlwatch/ - (not aware of any SaaS offer - self-hosted it is).

Yep, urlwatch is a good one too. This category clearly has a strong self-hosted tradition. With Site Spy, what I’m trying to make much easier is the browser-first flow: pick the exact part of a page visually, then follow changes through diffs, history, RSS, and alerts with very little setup