One great thing about the internet is the ability for anyone to access large amounts of data.  Data aggregators like FindTheData present lots of information in ways that are easy for students to understand,organized visually into easily readable charts.  FindTheData also gives students the opportunity to select entries to compare across certain domains later.  The database covers topics as diverse as prison demographics and known exoplanets all the way to logical fallacies.  Users can create a free account if they want to contribute to the database, but no login is required to view and manipulate the data.

Website: http://www.findthedata.com/
Subject: All subjects
Grade Level: 3-12
Price: Free