My algorithm is something I made up, and from memory it works like this:
1) Remove HTML, stem, remove stopwords etc
2) Sort unique words by popularity in the text
3) Split the original text on sentence boundaries.
4) Include each sentence that first mentions the next most popular word, until the summary is the maximum length requested.
http://news.ycombinator.com/item?id=1803020
Googling turns up http://sujitpal.blogspot.com.au/2009/02/summarization-with-l... which compares a few approaches.
Edit: Also http://stackoverflow.com/questions/2829303/given-a-document-..., which I think is by Dn_Ab who wrote the OP.
