Whether it is for alerting to Black Friday or other online specials, competitions, or availability of tickets, these types of extensions can be very useful.
Distill Web Monitor is not the only one out there, but it seems to be an overwhelming favourite on the Chrome store with over 200,000 installs, and an average rating of 4.6/5.0 stars. It allows for some really flexible monitoring as well as having many options for alerts.
The free version does have some limitations, especially for the cloud version (no browser needs to be active), but if you run the alerts on your local browser you can have up to 25 running, with alerts set down to 5 seconds. Mobile app versions are only for paid services.
Visualping is another very popular service, but it has a lot more restrictions on the free plan. Thing is, when you get down to hour and minute interval monitoring, many services charge for that as a cloud service. I still fondly remember the Check&Get application that you could install in the early 2000’s that used to do this. However, there is also a self-hosted open-source option called changedetection.io.
Track websites and pages for changes. Get email alerts, push notifications and slack/discord alerts on changes.