I mentioned open-source and Visual Studio in a previous post.

Today I would like to make it clear: It is possible to release extensions to Visual Studio under open source licenses.

First, some intro.  The VSIP SDK was originally introduced as a way to help developers extend Visual Studio.  It was (and remains) a free download, and works with VS2002 and VS2003.  For VS2005, the VSIP SDK has been "renamed" the VS SDK.  It accomplishes the same purpose, but for VS 2005.   You can download these things here.  (download requires passport authentication).

Here's the thing:  In the end-user license agreement for the original VSIP SDK, there was some language in there that essentially prohibited the release of extensions to Visual Studio, built with the VSIP SDK, under open-source licenses:

c. "Integration Code" means the software code that You develop through use of the VSIP-SDK to integrate Your Eligible Products into Visual Studio .NET Technology.

and

vii. You shall not expose or enable others to expose the functionality contained in the Integration Code in a manner that allows it to be accessed or used by any third party product. By way of example but not limitation, You may not disclose the Integration Code to third parties in source code form;

Those license terms clearly prohibit open-sourcing your extension, but they no longer apply.  That language is from the "old" VSIP SDK end user license agreement (EULA) for the old VSIP SDK. Those terms have been replaced.

In October 2005, when Microsoft released the new VS SDK, covering VS2005, we also introduced a new EULA that allows people to share Integration Code under a license of their choosing. (though the new EULA still prohibits copyleft). The new EULA language applies to both the VS SDK (used for extending VS2005) and to the VSIP SDK (used for extending VS2003).

Bottom line: if you are building an extension to Visual Studio, either 2003 or 2005, and want to release it under an open-source license, the new EULA ought to work for your purposes. But IANAL, so review it with your lawyer to be certain.

To see the new EULA, download the new VS SDK, and remember, it requires a passport login.