Make it easy for groups to extend the work they do in the classroom to times when they're off campus.
There are increasing options available for groups to meet and talk online, share files (including documents, presentations, spreadsheets, images, graphics, and more), and work on the same documents together (as in a wiki, or group-editable spreadsheet).
Two of the most-popular (and widely used) tools that facilitate collaboration are Google Groups and Zoho.
Collaborative literature reviews can be compiled using Delicious, a social bookmarking site that permits users to share "finds" (links to Internet sites) with one another, complete with category "tags" and comments. Begin by doing a search at Delicious for your own discipline or program area (here's one for physics), then construct an assignment that would have students refine their own list.
As you consider the idea of students using collaborative tools, you'll no doubt recognize that a profound shift is going on here. Our very conception of knowledge and the ways in which it is created is challenged and expanded by these tools. Knowledge in these environments is clearly something created by peer thinkers and researchers, rather than by individuals. You and your students might want to consider an assignment like the ones reported here, in which students are asked to work on actual Wikipedia entries: Identifying Best Practices for Student Wikipedia Projects and Using Wikipedia to Reenvision the Term Paper.