Chatter-api-qa-app is a project mainly written in ..., it's free.
A demo app that uses the Chatter REST API
This application demonstrates the 'un-authenticated user portal' use case. Try it out here. This application takes questions from users on a 3rd party site and posts them to a group in chatter. When it posts, the app posts on behalf of itself. It also pulls out the group feed and displays it.
Another example of this use case would be chatter exported to an HR intranet site where users who are not necessarily on chatter may ask questions, and users inside HR who are members of a group can answer questions. Replaces the typical list of email addresses and lets a team work a queue out of a group feed.