Uptime Robot: Free website monitoring, supports 50 sites, once every 5 minutes

Uptime Robot, founded in 2010, mainly provides website monitoring and started offering a paid version in January 2015. The free version supports 50 sites, detects once every 5 minutes, and saves 2 months of logs. The paid version supports 50 sites, detects once every 1 minute, saves 24 months of logs, provides 20 SMS notifications per month, and costs US$5.5 per month.

There are many monitoring points, see: https://uptimerobot.com/locations.php.

Monitoring types include http(s), ping, keyword, port, etc.

SMS notifications can be purchased separately, with prices of 50 messages/$15, 100 messages/$25, 250 messages/$55, and 500 messages/$100.
Support RSS, API, etc.

Uptime Robot official website : https://uptimerobot.com/pricing