Infocetera - Lotus Notes for the (Linux) masses (Score 1)

Infocetera comes pretty close to being a replacement for Lotus Notes and Exchange. It runs on most any platform and provides a rich suite of information management and group collaboration tools. Currently, it is not open source but much of the source is released (and will soon be under a BSD like license).

Disclosure: I am the developer of Infocetera

-Jeff McWhirter