Generic / multipurpose

• Datadog: https://www.datadoghq.com/ (SaaS, ~15 $ / month / host)

Status page applications

• Statuspage: https://www.statuspage.io/ (SaaS, 30 to 400 $ / month)
• Cachet: https://github.com/cachethq/Cachet (PHP)
• Stashboard: https://github.com/twilio/stashboard (Python on GAE)

Software

• New relic: http://newrelic.com/ (SaaS, 50 to 150 $ / month, free lite version available too)
• Sentry: https://github.com/getsentry/sentry (Python/Django)
• Hosted Sentry: https://getsentry.com/welcome/ (SaaS, 20 to 200 $ / month)