Chandler is an open source Note-to-Self Organizer designed for personal and small-group task management and calendaring. (Apache)

Collabtive is web based collaboration software. (GPL)

cyn.in is an Open Source Collaborative Knowledge Management Appliance that enables teams to seamlessly work together on files, documents and content in a secure central environment. (GPL)

Kablink Teaming boosts individual and team productivity by bringing people together to create, share, discuss and manage information. (CPAL)

MindTouch Deki is a web-based enterprise collaboration, wiki software and mashup platform. (GPL)

Spicebird is your one platform for many collaboration needs. It provides e-mail, calendaring and instant messaging with intuitive integration and unlimited extensibility. (MPL)