Collanos Workplace is a really interesting peer-to-peer application for creating ad-hoc project workspaces to store files, notes and other shared data. You can invite other users into your workspaces and maintain a common repository of documents.
Collanos is built on the Eclipse Rich Client Platform and is thus inherently cross-platform, like most teams today. Apple even featured Collanos in its list of OS X applications yesterday.
Sad to say, Collanos is not open source, but it is free of charge and promises to remain so forever.