TileMill - Completely customizable map tiles and interactive maps

TileMill gives you the ability to design and create your own custom map tiles, as well as static maps and maps of your data. You can use any of the many provided data sets (such as countries, geographic terrain, or population density) and add and use your own data in CSV format as well. You control all styles using CartoCSS, which is similar in syntax to regular CSS, and can style various elements based on the data in your CSV file. I love it because you can add and style anything you want, and it outputs many formats, including map tiles you can use with MapBox or host on your own, PDF, PNG, and SVG files. While it's a mapping tool at heart, I'm going to be experimenting with using it to generate charts and graphs for infographics next.