None have that I know of. Several have used Clojure.
Frankly, I would not recommend a startup use it. Although we continue to tweak our version, we haven't bothered to release a new version for a long time. And I doubt there is much in the way of libraries or integration with other stuff.
I noticed awhile back that user rougefeta used Arc or something strongly resembling it for their webapp they were applying to YC with, have any others also used Arc-based webapps to apply to YC?
rougefeta seems to have taken the site down though.
Arc is in much the same state as essays. YC has grown so big that I don't have much time to work on either, except as related to YC (i.e. essays about startups, and Arc tweaks to support HN). But one way or another I will work on both again.
Calling Python from Arc with wrappers that return Python data structures as s-exps seems like a workable hack to get libraries in Arc, at least the ones written in a functional style.
Most libraries needed for web apps might be in Javascript.
Frankly, I would not recommend a startup use it. Although we continue to tweak our version, we haven't bothered to release a new version for a long time. And I doubt there is much in the way of libraries or integration with other stuff.