Free! PER SQUARE MILE: This interesting mashup of geography, ecology, demography, and urban planning is an excellent resource for seeding conversation around complex social studies topics and promoting critical thinking. Per Square Mile first jumped on our radar with this thought-provoking hypothesis that income inequality is linked to urban trees and these pictures as supporting evidence. Author, Mr. Tim De Chant, is a self-labeled journalist, designer, and researcher -- all of which are apparent in his blog posts and curations. After you Google Map your local neighborhoods for signs of income inequality, also check out this collection of simple graphics that explain the world's population density.