[0].http://underscorejs.org/docs/underscore.html
Some more links:
Backbone.js - http://backbonejs.org/docs/backbone.html jQuery 1.6 - http://robflaherty.github.io/jquery-annotated-source/
http://gitlet.maryrosecook.com/docs/gitlet.html
Small Lisp interpreter in Python:
http://khamidou.com/compilers/lisp.py/
If you are interested in annotation formats other than side-by-side, many Jupyter or IPython notebooks would qualify, for example Peter Norvig's programming exercises:
Lacks an actual description of the grammar/semantics of the implemented language.
Should this work:
((lambda (foo) (+ foo 3)) 39)
(begin (define foo (lambda (a) (lambda (b) (+ a b)))) (define bar (foo 20)) (print (bar 22)))
but of what language? If you don't document the language you are trying to implement, we don't know what code should be valid. It's especially easy to implement something if you don't say what you are trying to implement...