There are many platforms out there that claim to implement groupware these days.
One of first systems on the market was Lotus Notes/Domino. It’s still available and it has defined the category for this kind of software.
If you decided to use that system in the past, you did it right: It’s still there, fully supported by IBM. All your investments in this platform are save. The system platform is even now evolving as a rapid prototyping framework for the Web 2.0 world. if you need something not build in, you can develop custom applications or get others do that for you.
New features get implemented in new releases all the time. This adds value for your business. Automatically. Your people do not need to change the way they work. Notes Applications will adopt to what you need. They evolve into something fitting better into your business.
There is even a new paradigm available to use it in new projects: XPages gives Notes Domino developers a new way to be highly productive using the Notes/Domino system.
Besides the standard built in features like mail, calendar, contacts and to do’s, new ways of working together can be implemented.
Even if you do not have the time to develop anything new from scratch, there is an open source repository that delivers Apps for free: OpenNTF.