Mapsicon is a free collection of maps for every country in the world, available in 11 different sizes, from 16x16 pixels to 512x512 pixels.
Icons are in PNG-8 bits with alpha transparency (basically, the background is transparent) and are in a black (#000) color. If you need them with a specific color, we can provide it to you in 48 hours for a fixed amount. Contact me for more information.
All the icons have been optimized to have the smallest possible size, using PNGOUT, AdvPNG, Pngcrush and OptiPNG (through ImageOptim). Basically, 1024 x 1024 icons weight around 12 KB.
The maps are named after the official two-letter country codes defined in ISO 3166-1. Basically, that means the United States are named US, Canada is named CA - you get it.
Some countries have been intentionally omitted, either because they have no official maps yet or because they are islands that are so small that they can not be drawn. The omitted countries are Micronesia, Jersey, Marshall Islands, Northern Mariana Islands, Palestinia, Tuvalu and United States Minor Outlying Islands.
Do what you want with them as long as you mention me in your project. Please don't resell them - I forbid it!
If you need to access all those countries programmatically, here is a gist containing all the countries as well as their ISO code: https://gist.github.com/djaiss/2938259