Simon
www.dejal.com/simon
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
Dejal Simon is the essential site monitoring tool for Mac OS X. It checks servers for changes or failures, and notifies you via e-mail, sound, speech, Twitter, or other means. You can use it to track updated sites, and to alert you when an important server goes down or recovers. Developed in Cocoa, it is a native Mac OS X application with an intuitive and attractive interface.