[stugrey]

Their TileMill software is great. You don't have to sign up to have them serve your maps, you can use all of their tools to generate your tilesets and do what you like with them (obviously attributing your data set source). The MapBox iOS SDK is another stand out contribution from these guys.

[hnha]

Some of their free offerings are so great that i sometimes worry they might be (maybe unintended) in a "embrace, extend" phase, extinguishing competitors left and right. Their uprise is insane.