Open Source is often used interchangably with FreeSoftware.
However, it carries a secondary meaning. Whereas FreeSoftware is a political ideal from RichardStallman, OpenSource is part of EricRaymond's rebranding away from a Stallmanite interpretation. (See more on HackingIsASickMachoCulture)
TimOReilly, roughly in the Raymond camp, defines it like this : OpenSource/ParadigmShift
How free became open : (though http://www.media-culture.org.au/0406/02_Coleman-Hill.html (though compare CommoditizeYourComplements for alternative explanation of IBM's engagement with FOSS)
I commented that the M/C article misses a couple of points.
1) "FreeSoftware" from RichardStallman has always been a political idea. And one which can be understood as straight up "left" ie. a belief in communaly accessable resources rather than private property.
So it's slightly strange to say FOSS is broad enough to have been "adopted" by the left. It was theirs originally.
2) The confusion arises because of the move to the "right" - ie. from "free software" to "open source" - which was a deliberate political action by some people within the movement long before IBM et al became involved.
Look at the explicit writings of key people like EricRaymond and TimOReilly. Remember the terms "Cathedral" and "Bazaar" don't refer to FOSS vs. proprietory software. They refer to bottom-up, distributed vs. top-down, centralized ways of developing. Raymond is refering to market economies vs. centrally planned ones. In "Homesteading on the Noosphere" he is trying to assert the primacy of a notion of private property as the way to understand the political economy of FOSS.
3) Lessig doesn't strike me so much as a "Centrist" as someone moving to the left in this domain. Note he's now joined the Free Software Foundation, so is a member of the cathedral.
It's because they miss the dynamic history of this story, that M/C seems to think that FOSS is just a broad idea suitably adaptable to different groups. In fact it's a left idea that's been "spun" for the right.