changedetection.io
The open source option, and the strongest choice if you are willing to run it yourself.
- Describes itself as "a fully opensource project", with the repository linked from its own site.
- Hosted subscription listed at $8.99/mo, "Cancel anytime - 5,000 URL watches included".
- Re-check intervals from 5 minutes, and 85+ notification types.
Best for. Technical users who already run a server and want to own the data and the deployment.
The cost. The cost is attention rather than money. You are running the monitor, which means you are also the one who notices when it has quietly stopped.