Scintilla is something I've been working on for a while at NPG. We opened it up earlier this week - if you want a full description, read the post on Nascent.
It's a bit like The Hype Machine, in that it aggregates content from sources focused around a particular topic (in this case science). It's a bit like Google Reader in that you can choose to subscribe to particular sources and see new items from those sources in your reading list (but it takes the approach Christian Montoya described today, in that there's no 'unread items' to keep up with). It's a bit like last.fm, in that it recommends items to you based on what other people liked (though it uses active ratings rather than passive). It's a bit like del.icio.us, in that you have a social network and can recommend items directly to people and groups. It's built on Drupal, with a few core hacks and some custom modules, as well as SimplePie, Solr for indexing, memcached and APC for speed. If you'd like to try it out, feel free to browse around or create an account (you can use your nature.com login if you already have one).