3. MediaWiki
It is the volunteer-supported foundation behind Wikipedia; that alone makes it a life-saving tool for about a billion college students. MediaWiki allowed Wikipedia to reach its lofty heights ,and while it is often criticized for being somewhat difficult to learn, that certainly does not stop it from being the tool that allows thousands to edit and share information (whatever the consequences may be). Also, while we’re on editing, consider WordPress, which has given so many people the power to blog, it is downright uplifting (and, depending on the writer, sometimes hideously annoying).