Euan Adie's provided two very useful science-related web features recently. The first is in Postgenomic, which now allows you to look up blog citations for a paper using the DOI, which means any journal article can link to relevant blog conversations - here's an example: see the 'Referenced by' section. This is a much better solution to tracking distributed conversations than re-posting articles to a central blog for commenting.
The second is a file storage service for Connotea. Unlike CiteULike, which lets you upload files to attach to each citation and stores them on a central server, this uses a Greasemonkey script and the box.net API, making the file storage service almost independent of Connotea (though it still uses a central service to place the files on box.net). A tag on each Connotea item stores the box.net file ID, and Greasemonkey writes the links to upload and download files into the page.