I had the delightful opportunity to chat with an old friend, Juergen Geck who is CTO of my former employer Open-Xchange. We've known one another since his days at SUSE (now part of Novell).
Open-Xchange has made a great deal of progress since my time with them. They've developed a hosted version of their collaborative application software (available as an offering through several hosting companies), updated their packaged server solution, added a new "appliance" edition and have announced the support of iPhone and Mac OS X-based systems to the long list of supported clients.
If you're interested in a cost-effective, comprehensive collaborative solution, Open-Xchange would be something interesting to review.