Shared Source is a term, primarily used by Microsoft, to define a pseudo-open source form of code sharing.
With shared source, the source code to a particular piece of software is made available to reference. However, unlike most open source licenses, the authors maintain strict control over the use of that code once it has been read. For example, many shared source licenses permit only academic use of the source code, or permit reuse of the code only for non-commercial use, or permit reading but no deriving from the code base.
Proponents of shared source see it as a step forward from purely proprietary development. In the particular case of Microsoft, their shared source initiatives often permit developers to see source code they otherwise would have no access to. This permits better integration, debugging, interaction, and standardization among products.
Opponents of shared source often see it as too little, too late. Many outspoken open source advocates consider the shared source licenses as illusionary. That is, they give the impression that they are making the source code freely available, when in fact there are critical restrictions on the code's use. There is also growing concern the the shared source program is being used to harvest the names of developers for future legal action if they ever participate in development project seens as competitive to a Microsoft product.
Perhaps the most notable shared source license is that covering Rotor, the shared source implementation of the Microsoft .NET CLI. This implementation is freely available, including source code, as a reference guide. The license explicitly permits non-commercial use of the source code, including derived works. It explicitly forbids use of the code, or derivatives, in any commercial software *or* open source software. (One of the provisions is that derived works must use a license that is at least as restrictive as the original shared source license.)
