The Best Free and Open Source Data Mining Software
If I may plug the 140kit (http://140kit.com) which is an open source Twitter mining and analytics solution that uses the streaming API to make downloading millions of Tweets in easily accessible formats quick and easy for anyone.(doh, it looks like its down for some reason. Well, when it's back up- its there. I'm sure you could find the github repo and run it yourself since it is open source). Written in Rails.