Q:

Do any "major" frameworks make use of monkey-patching/open classes

I am curious about the usage of the feature known as open classes or monkey-patching in languages like e.g. Ruby, Python, Groovy etc.  This feature allows you to make modifications (like adding or replacing methods) to existing classes or objects at runtime. 
Does anyone know if major frameworks (such as Rails/Grails/Zope) make (extensive) use of this opportunity in order to provide services to the developer?  If so, please provide examples. 

A:

Rails does this to a (IMHO) ridiculous extent.