Free Software vs. Open Source
As has already been described in chapter 1, Free Software and Open Source have more or less the same aims but differ in their philosophy and values:
The fundamental difference between the two movements is in their values, their ways of looking at the world. For the Open Source movement, the issue of whether software should be open source is a practical question, not an ethical one. As one person put it, ``Open source is a development methodology; free software is a social movement.'' For the Open Source movement, non-free software is a suboptimal solution. For the Free Software movement, non-free software is a social problem and free software is the solution. gnu.org
This said, these two movements basically use the same practical methods while having different principles. The Open Source movement (and later the Open Source Initiative) was initially meant as a "marketing" initiative for Free Software partially caused by the ambiguity of the term "free" and also of its implications. For example can "free" be understood as "free of costs" ("Free as in beer" or "Free as in lunch" as Microsoft once adapted the phrase for a sponsored lunch) instead of the adjective "free" derived from "freedom" (as used by the FSF and GNU).
The Free Software movement also sometimes appears to be quite aggressive which in the end and motivated by Netscape's move to put the code of their browser engine under a FS/OS license caused the formation of the OSI around Bruce Perens and Eric S. Raymond in 1998:
We realized it was time to dump the confrontational attitude that has been associated with "free software" in the past and sell the idea strictly on the same pragmatic, business-case grounds that motivated Netscape. We brainstormed about tactics and a new label. "Open source," contributed by Chris Peterson, was the best thing we came up with. opensource.org
In an interview in Feb. 1998 ESR also brought up that the whole philosophical focus of the Free Software movement is in his eyes a problem for motivating companies to adopt the whole concept:
[...] but in the battle we are fighting now, ideology is just a handicap. We need to be making arguments based on economics and development processes and expected return. salon.com
On the other hand "Open Source" without the definition of the OSI simply defines a software as "open source" if the user can access the source code. Here both projects have the same problem since both terms without the clear definitions can and are understood under different meaning than those specifically defined by the two organisations.
To make it clear to the customer that she just bought open source software the OSI has also created a certificate. This certificate can be applied on software licensed under one of the licenses approved by the Open Source Initiative.
This said, both movements don't seem to work again each other but more on different battle grounds for a common goal. According to ESR the rhetorics of the FSF and esp. of Richard Stallman seem to work better on programmers/hackers while the arguments of the OSI are more targeted at companies. But there are also some people who think, that the Open Source movement overshadowed the efforts of the FSF and GNU more than it is fair as Bruce Perens stated in a letter announcing his return to the Free Software movement in 1999