It's open-source: https://github.com/caolan/async

Yes, I've seen it on HN before and it's the bee's knees.

I meant to link that. Now it's linked in the post, and it's here: https://github.com/caolan/async

Funny story, I found this particular lib the same day as Daniel wrote some code that was an exact drop-in replacement for its 'parallel' function. So he says 'Yes, let's use that one, as I understand and approve of its approach.'