Yowl provides a set of Ajax interfaces that web applications can call to display notifications to the user. The advantage of calling to Yowl rather than managing the notifications directly is that Yowl allows the user to control the style of the messages, or even whether the messages should appear at all.

Yowl is inspired by the Growl system for Mac OS and will integrate with it.

Initial work for Yowl took place at the London Hackday 2007.