I love the annotated source code, but I wouldn't necessarily recommend the Underscore library code itself as learning material to budding developers - it's optimised for size and performance, not legibility and idiomatic style. I remember being a budding JS dev once upon a time, and being quite perplexed by it :)
Also interesting, even if it has become unfashionable these days, Backbone's annotated source: <a href="http://backbonejs.org/docs/backbone.html" rel="nofollow">http://backbonejs.org/docs/backbone.html</a><p>And the tool to make these: <a href="http://ashkenas.com/docco/" rel="nofollow">http://ashkenas.com/docco/</a>
For beginners annotated code of highly optimized library might become confusing. Try reading Mary Rose Cook's annotated code of 2 JS games and a Git clone
<a href="http://annotated-code.maryrosecook.com/" rel="nofollow">http://annotated-code.maryrosecook.com/</a>
Probably a bit long in the tooth now:<p><a href="http://robflaherty.github.io/jquery-annotated-source/" rel="nofollow">http://robflaherty.github.io/jquery-annotated-source/</a>