I know that the discussion on the "right" licence for open source occupied a significant portion of internet bandwidth and so is here the reason why, in my opinion, FreeCAD should have this one.
I chose the LGPL for the project and I know the pro and cons about the LGPL and will give you some reasons for that decision.
FreeCAD is a mixture of a library and an application, so the GPL would be a little bit strong for that. It would prevent writing commercial modules for FreeCAD because it would prevent linking with the FreeCAD base libs. You may ask why commercial modules at all? Therefore Linux is good example. Would Linux be so successful when the GNU C Library would be GPL and therefore prevent linking against non-GPL applications? And although I love the freedom of Linux, I also want to be able to use the very good NVIDIA 3D graphic driver. I understand and accept the reason NVIDIA does not wish to give away driver code. We all work for companies and need payment or at least food. So for me, a coexistence of open source and closed source software is not a bad thing, when it obeys the rules of the LGPL. I would like to see someone writing a Catia import/export processor for FreeCAD and distribute it for free or for some money. I don't like to force him to give away more than he wants to. That wouldn't be good neither for him nor for FreeCAD.
Nevertheless this decision is made only for the core system of FreeCAD. Every writer of an application module may make his own decision.
FreeCAD uses two different licenses, one for the application itself, and one for the documentation:
See FreeCAD's debian copyright file for more details about the licenses used by the different components found in FreeCAD
Private users can use FreeCAD free of charge and can do basically whatever they want to do with it: use it, copy it, modify it, redistribute it. They are always master of their data, they are not forced to update FreeCAD, change their usage of FreeCAD. Using FreeCAD doesn't bind them to any kind of contract or obligation.
Can use FreeCAD freely, for any kind of private or professional work. They can customize the application as they wish. They can write open or closed source extensions to FreeCAD. They are always master of their data, they are not forced to update FreeCAD, change their usage of FreeCAD. Using FreeCAD doesn't bind them to any kind of contract or obligation.
Can use FreeCAD as the groundwork for own extension modules for special purposes. They can choose either the GPL or the LGPL to allow the use of their work in proprietary software or not.
Commercial developers can use FreeCAD as the groundwork for their own extension modules for special purposes and are not forced to make their modules open source. They can use all modules which use the LGPL. They are allowed to distribute FreeCAD along with their proprietary software. They will get the support of the author(s) as long as it is not a one way street.
The following is no more applicable since version 0.14, since both FreeCAD and OpenCasCade are now fully LGPL.
Up to Version 0.13 FreeCAD is delivered as GPL2+, although the source itself is under LGPL2+. Thats because of linkage of Coin3D (GPL2) and PyQt(GPL). Starting with 0.14 we will be completely GPL free. PyQt will be replaced by PySide, and Coin3D was re-licensed under BSD. One problem, we still have to face, license-wise, the OCTPL (Open CASCADE Technology Public License). Its a License mostly LGPL similar, with certain changes. On of the originators, Roman Lygin, elaborated on the License on his Blog. The home-brew OCTPL license leads to all kind of side effects for FreeCAD, which where widely discussed on different forums and mailing lists, e.g. on OpenCasCade forum itself. I will link here some articles for the biggest problems.
We first discovered the problem by a discussion on the FSF high priority project discussion list. It was about a library we look at, which was licensed with GPL3. Since we linked back then with Coin3D, with GPL2 only, we was not able to adopt that lib. Also the OCTPL is considered GPL incompatible. This Libre Graphics World article "LibreDWG drama: the end or the new beginning?" shows up the drama of LibreDWG project not acceptably in FreeCAD or LibreCAD.
The incompatibility of the OCTPL was discussed on the debian legal list and lead to a bug report on the FreeCAD package which prevent (ignor-tag) the transition from debian-testing to the main distribution. But its also mentioned thats a FreeCAD, which is free of GPL code and libs, would be acceptably. With a re-licensed Coin3D V4 and a substituted PyQt we will hopefully reach GPL free with the 0.14 release.
In the Fedora project OpenCasCade is listed "non-free". This means basically it won't make it into Fedora or RedHat. This means also FreeCAD won't make it into Fedora/RedHat until OCC is changing its license. Here the links to the license evaluation:
The main problem they have AFIK is that the OCC license demand non discriminatory support fees if you want to do paid support. It has nothing to do with "free" or OpenSource, its all about RedHat's business model!