Wikipedia allows your students to collaborate with an enormous extramural community. You can assign a single article or use the resources developed by the Wiki Education Foundation to design an entire course.
Google Docs combine the familiar workspaces of word processors and spreadsheets with WYSIWYG live online editing. With Google Docs, students can work together on group assignments, share class notes, or fill out study guides.
Hackpad is a WYSIWYG text pad that can be easily embedded in other platforms. You can add it to a blog or even an LMS to create a collaborative space for your students.
Wikispaces offers a WYSIWYG editor, and also a simplified course management system. Students can share files, work on structured projects, and complete events.