Ask HN: Libraries for classification of text

I've searched fairly hard, but I'm finding it hard to locate exactly what I want. Maybe it doesn't exist, and maybe I just need to adapt something that's there, or write my own. But I thought I'd ask.

I have a bunch of text items that have had classifications applied to them. These classification aren't exclusive - each item may be in one or more classes. Perhaps it's best to think of them as tags.

What I want is a system that has a stab at applying these tags to previously unseen items, based on the tags of existing items. I know that such things exist, but they all seem to be poorly performing, proprietary, hard-to-use, free-but-closed, or otherwise just not quite what I want.

Can I take suggestions from the floor? Is this a problem you've dealt with, and what did you use?

TIA.