Moonitor
Uptime monitoring for a one-off price on AppSumo: 20 monitors at 60-second checks, SSL checks, all alert channels and branded status pages.
Upptime
Free, open-source uptime monitor and status website that runs on GitHub, checking your pages every 5 minutes. It needs a GitHub repository.
Uptime Kuma
Free, open-source uptime monitor and status pages that you host yourself, with checks as often as every 20 seconds and 90+ alert services.
Cronitor
Cron job and heartbeat monitoring, plus website and API checks, on a free plan for small projects: 5 monitors, email and Slack alerts.
Atlassian Statuspage
A free public status page to keep customers updated during outages, with email, Slack and Teams notifications. It doesn't monitor your site itself.
Instatus
A status page with uptime monitoring built in: 15 monitors at 2-minute checks and a public page for up to 200 subscribers, free with no card.
Better Stack
Uptime checks, cron heartbeats and a status page on a free plan for personal projects, with Slack and email alerts. Paid plans add on-call tools.