Google now offers Google Scholar, a search engine for online papers. The CS community has had something like this for a while in the form of CiteSeer. Google Scholar, unlike CiteSeer, does not archive the papers, but it seems to get its data from a much wider selection of sources : not just personal webpages, but also arXiv, the PhilSci archive, publisher’s webpages, and databases such as ingenta. You get links to all the available versions of the document (preprint, published version) as well as searches for all papers that cite a given paper. Cool.