Zotero, a personal research assistant, directly into your browser. When I was told about it, was quite impressed. I've been dreaming of disposing of the ugly looking Endnote - and the feature list of zotero seemed pretty attractive!

Automatic capture of citation information from web pages
Storage of PDFs, files, images, links, and whole web pages
Flexible notetaking with autosave
Fast, as-you-type search through your materials
Playlist-like library organization, including saved searches (smart collections) and tags
Platform for new forms of digital research that can be extended with other web tools and services
Runs right in your web browser
Formatted citation export (style list to grow rapidly)
Free and open source
Integration with Microsoft Word and OpenOffice

Not that easy, however. Zotero is great when it comes to extract and format citation data from a webpage, and maybe add notes and tags, but it slows down considerably when you import large libraries of citations into it (the sort of libraries we are all likely to have in endnote or other similar softwares). I havent tried it myself yet (sorry too risky -  my firefox on the mac is already bogged down by something at times), but you can check out some reviews here, here and here. ..... so, altough a major step in the right direction, i guess i'll have to use both for a little while....