Ping?

Full Disclosure, this is a Cisco product and I work there.

ThousandEyes Internet Insights offers a public outage map at https://www.thousandeyes.com/outages/ that is MASSIVELY more accurate (and faster to detect) than Down Detector because its based on actual telemetry and not on crowdsourcing.

The snarky but honest response is HackerNews/Reddit. But if you're trying for something that actually checks the sites you point it at instead of relying on users reporting "issues", a buddy of mine runs https://onlineornot.com/ and it seems generally more reliable.

My extensive DDoS bot network... er I mean my distributed website validation platform!