as others have pointed out, their map authoring tool, TileMill is open source, as well as many other projects (iD for editing OSM, mapnik is heavily authored by MapBox engineers, etc). But it goes quite a bit beyond even that. Their tile hosting software is mostly open source as well, which they call TileStream. Without too much trouble you can actually recreate the bulk of their SaaS hosting offering using their open source code base. So their SaaS offering is pretty close to fully open source. They're starting to move into premium data (ie satellite data after a natural disaster), so obviously that stuff isn't going to be open, but it is pretty impressive how much of their entire stack they develop on github.