Mendeley is starting to look very promising. They have a London-based team of academic founders; chairman Stefan Glänzer, and a bunch of KDE developers who have put together a decent Qt client for Linux - alongside the Windows and OS X versions.

There's a website with a basic social network with groups and contacts (still missing a way to invite people), and the important bit: when you add papers to your local collection (by importing PDFs, BibTeX or RIS) they sync with your online collection, which is private by default but can be shared with other members. In other words, collect some contacts and share your reading lists, with no extra effort than adding a PDF file to your local collection.

When you import a PDF file*, it detects the DOI and looks up the metadata from CrossRef. It even does a reasonable job of parsing the references out of the bibliography... though it's not perfect, and of course the references don't have DOIs (argh, PDFs).

Mendeley Desktop is still a problematic beta version and not nearly as slick as Papers, but it is cross-platform. The main problem I can foresee is that bibliography managers are going to need a plugin architecture (see the feature request for a pluggable context pane that I posted on the Papers forum the other day, for example, or the multitude of add-ons available for Songbird) and these plugins will need to be cross-platform too, which - to have the greatest possible chance of being created - probably means Javascript and XUL or WebKit.

* PubGet is a good way to get straight to the PDFs if you're collecting lots of articles - there's even a mobile version.