I think this is more of an underutilized features rather than an uncommon one, but I really like Extension Methods. They allow you to "attach" a method to a class that you otherwise can't or don't want to modify.
For example, rather than:
You can write:
In Xojo you'd do this with the Extends keyword in the global method definition:
Public Sub Foo(Extends FrameworkClass As c)
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