Disclaimer: Read this post with the knowledge that I really only know enough about Ruby to be dangerous.
Hmm, no I don't think there's anything particularly standout about test/unit. I just really REALLY like unit testing and I think I didn't understand what you meant about notable libraries.
Oh, actually, now that I think about it, optparse is pretty sweet and I've never really seen anything like it other languages I've used (though I'm not really up on Python, so it may have something similar) RDoc here: http://www.ruby-doc.org/stdlib/libdoc/optparse/rdoc/index.html
Back to Object mods: Well, you know, it's not too hard to do change localization in Object with the alias method. Like:
alias :meth, :old_meth def meth ... end
and then when you're done with being a clever bugger:
alias :old_meth, :meth and Bob's your Uncle, back to normal.
I don't think popping Object open and changing it around is the flip side of Python Magic Methods though. Read the following section with the knowledge that I am no Ruby expert and know only of Python from mythical tales of its courage in battle:
From what I've used of Ruby, it would seem that the actual flip side of Python "magic methods" is just using some standard method names, like "to_s" or "each" that behave like they should for the Object they're inside of. I think the "magic" is typically accomplished with mixins, Enumerable being the example used in the Pickaxe where you define "each" to get a bunch of the iterators and "<=>" if you want the rest (I think defining "<=>" and including Enum also gives you Comparable, but you might need to add it explicitly) and then if you want a special version of one of the methods the mixin adds, you just go ahead and change it.
I'm gonna shut up now because if I keep talking, I'm probablly going to misrepresent something and start a flamewar.
Oh, actually, now that I think about it, optparse is pretty sweet and I've never really seen anything like it other languages I've used (though I'm not really up on Python, so it may have something similar).
http://docs.python.org/lib/module-optparse.html -- need I say more? optparse existed in Python under name of Optik since 2001: http://optik.sourceforge.net/ and given apparent similarities in API, I even doubt whether Ruby one is clone of Python one...# Seo Sanghyeon