Wisper: A micro library providing Ruby objects with Publish-Subscribe capabilities

• Decouple core business logic from external concerns in Hexagonal style architectures
• Use as an alternative to ActiveRecord callbacks and Observers in Rails apps
• Connect objects based on context without permanence
• Publish events synchronously or asynchronously

Source: krisleech/wisper: A micro library providing Ruby objects with Publish-Subscribe capabilities

Nice, and better than rolling your own.