The plugin my current employer uses is so well integrated that I don't even know its name. (I suspect it may be developed internally)
At a past job, we used Entrust [0] and I'm aware of Virtru [1] as well.
Edit: I forgot about Sharepoint, which also sort-of fills the ACL document-sharing niche. (though I'm less certain about whether it uses encryption to enforce its access policies)
