<p>I'm looking for an existing library to summarize or paraphrase content (I'm aiming at blog posts) - any experience with existing natural language processing libraries?</p>

<p>I'm open to a variety of languages, so I'm more interested in the abilities &amp; accuracy.</p>

Your getting into really far out AI type domain. I have done extensive work in text transformation into machine knowledge mainly using Attempto Controlled English (see:
http://attempto.ifi.uzh.ch/site/
), it is a natural language (english) that is completely computer processable into several different ontologies, such as OWLDL.
Seems like that would we way overkill though...
Is there a reason for not just taking the first few sentences of your blog post and then appending an ellipse for your summary?