I've been playing a lot with Dojo
lately, and have been very impressed by its elegant publish-subscribe
system. Basically, any object can publish an event, and any other object can
subscribe to it. This creates an incredibly flexible notification
architecture that's completely opt-in.
The system has elements of Aspect Oriented Programming (AOP), as well as the
Observer pattern. Its power, however, is in the fact that an individual
object does not need to implement any specific interface in order to act as
either a Subject or an Observer; the system is globally available.
Being a developer who recognizes good ideas when he sees them, of course I
decided to port the idea to PHP. You can see the results on github.