For an idea of what you can do with TileMill, the Chicago Tribune demonstrated some of its capabilities as part of its Making Maps series using PostGIS, Mapnik, TileMill, and Google Maps. Demos of TileMill in action can also be viewed on MapBox’s Maps page.
TileMill has also released a crash course to help new users walk through using the open source application. The course reviews:, Importing a basic spreadsheet, Styling a basic point map, Adding tooltips and legends, and Exporting maps and sharing them on the web.
TileMill uses open source libraries: Mapnik, node.js, backbone.js,express and CodeMirror. CSS styling is utilized by TileMill with its map styling language Carto to render beautiful that can either be exported as image and PDF files or uploaded and displayed online.