Can't immediately find any IRS documents defining the term, but here is the definition from Black's Law Dictionary:
> Structures (such as roads or dams) built by the government for public use and paid for by public funds.
There is also a maxim of statutory interpretation in law that a word it known by its fellows. In this case, that would mean preferring an interpretation of "public works" that yields a certain kinship between that term and "public building" and "monuments." This would seem to militate in favor of the Black's/Wikipedia definition. Note also that these three are followed by "lessening the burdens of government" which might be another hint. (I.e., organizations that support public works are deserving of tax exempt status because they lessen the burdens of government.)
I can also tell you anecdotally that, at least among lawyers, the term is very well known and is definitely widely understood as having the definition given by Black's and Wikipedia. There may be a creative argument to be made that an open source project is a public work (could make a fun law review article!), but I doubt it would go very far.
