Seth Gottlieb has published an excellent whitepaper summarizing 15 different open source projects. He also includes a summary of how to go about evaluating open source offerings.

Seth offers a short list of offerings for each of the following usage scenarios:

Brochure Site
Online Periodical
Collaborative Workspace
Wiki as Collaborative Workspace
Online Community

The format is similar to the presentation he gave at KM World but the format obviously lets him go into much greater detail.