Mancomm Performance Inc of Montreal (with whom I’m associated), have recently developed a calendar function for the elgg learning landscape system. The calendar plug-in allows multiple users to post events to a common calendar and in typical Elgg fashion restrict access by group, community or public/private. In the spirit of the open source community, Mancomm paid for the development of this software but has released it under the GPL. From the elgg developer blog:
Yes, it is possible to be a small company, use open source software, give back to the community and still be profitable.