The team behind the open source Zotero citation management software has been actively developing “sync” functionality for quite some time now. It allows you to work with your references collection from multiple computers by storing the latest copy of your collection on the Zotero servers.
An optional part of the sync system allows you to store PDF files and such like in your collection (full-text copies of papers etc.). Zotero provides 100MB of storage free to everyone, but towards the end of 2009 they introduced an optional premium service, in conjuction with Digital Scholar. Read more Zotero premium service