TileMill is a lot of fun; you can download it and be making custom maps in just a few minutes. There's a lot of very sophisticated open source cartography hiding under the covers, not to mention nodejs wizardry. They've done a great job of packaging it all and making an easy to use tool for map-makers without requiring a lot of GIS or OSGeo expertise.

One particularly nice option is the ability to "print" your pre-rendered map to an MBTiles bundle for serving in a slippy map on the web.