From the website:
Crabgrass is a software libre web application designed for social networking, group collaboration and network organizing. Our goal is to create communication tools that are tailored specifically to meet the needs of bottom up grassroots organizing. […] Crabgrass is a secure alternative to for-profit social networking and organizing platforms.
From the about page (original site is now defunct, archived copy provided):
What Crabgrass Does
Crabgrass enables social change organizations to get things done, get the word out, collaborate and network.
To get things done, Crabgrass provides tools like task list management, meeting scheduling, asset management, wiki, and decision making.
When it is time to get the word out, Crabgrass allows groups to create a customized home page where a group can list their event calendar, blog postings, and other public content.
To help collaborate , Crabgrass allows users to rate content, add keywords to content, comment on content, annotate content, alert others to content you need their feedback on, and track participation and revisions. Additionally, Crabgrass comes with many tools for democratic decision making.
Since no group is an island, Crabgrass allows groups to create networks with other independent groups. These networks provide a place to work together on shared content and make decisions. Alternately, you can share a single document if that is the extent of the cooperation (in development).
Among the decision making tools are approval voting and ranked voting. Among the features are personal messages, discussions (public or private forums), wikis, file management, task lists, privacy, and security.
Crabgrass is open-source and its source code is available on 0xacab.
Crabgrass is developed Riseup Labs based in Seattle, WA (US).
Update: The site for Crabgrass is no longer viewable. In order to view the site, view an archived copy.