One thing I find myself doing (probably more often than I should) is adding a reference to my Zotero library for an item in the Biodiversity Heritage Library (BHL). BHL doesn't have article-level metadata (see But where are the articles?), so when I discover a page of interest (e.g., one that contains the original description of a taxon) I store metadata for the article containing that page in my Zotero library. Typically this involves manually step back through scanned pages until I find the start of the article, then store that URL as well as the page number as a Zotero item. As an example, here is the record for Ogilby , J. Douglas (1907). A new tree frog from Brisbane. Proceedings of the Royal Society of Queensland 20:31-32. The URL in the Zotero record http://www.biodiversitylibrary.org/page/13861218 take you to the first page of this article.
One reason for storing metadata in Zotero is so that these reference are made available through the bioGUID OpenURL resolver. This is achieved by regularly harvesting the RSS feed for my Zotero account, and adding items in that feed to the bioGUID database of articles. This makes Zotero highly attractive, as I don't have to write a graphical user interface to create bibliographic records. BHL have their own citation editor in the works ("CiteBank"), based on the ubiquitous Drupal, but I wonder whether Zotero is the better bet -- it has a bunch of nice features, including being able to sync local copies across multiple machines, and store local copies of PDFs (synced using WebDAV).
For fun I've created a Zotero group called Biodiversity Heritage Library which will list articles that I've extracted from BHL. At some point I may investigate automating this process of extracting articles (using existing blbiliographic metadata mapped to BHL page numbers), but for now there a mere 27 manually acquired items listed in the BHL group.