Pingthat: website monitoring service under stablehost

Pingthat, a free (paid) monitoring service launched by stablehost, supports http(s), ssh, dns, tcp, smpt, etc.

Official website: https://www.pingthat.com/

Supports 3 notification methods: Email, SMS, and custom URL.
An email activation link is required. The free version supports adding 2 services (during the promotion period, there is no limit on the number for the time being).
There are servers in Asia, the East and West Coast of the United States, and Europe.
The minimum monitoring frequency can be set to 1 minute.