Exchange alternatives / groupware - experiences? recommendations?

Our small company is considering to install some groupware. there´s of course ms exchange which works well with outlook, but there are also other solutions and we would prefer to have no Windows box in our rack.

our requirements:
- shared mail accounts
- document sharing
- no cloud computing, we´ll host it ourselves
- must work on Linux
- should, but need not support Outlook (or vice versa?)
- if paid solution, must be a onetime payment, payment for premium support acceptable

nice to have: support or integration of
- knowledge base
- bug or issue tracker
- crm

My personal dream would be an amalgam of groupware and software management system that also supports Eclipse and SVN, but I guess you can´t have everything.

Any products you have experiences with that I should consider evaluating?
What´s your favorite groupware and why?

Doesn't Oracle have a replacement for Exchange?

had a glimpse at Oracle Beehive, but I´m not quite sure if it´s a replacement. It definitely can be attached to Exchange.