I honestly don't know who is right or wrong from a legal perspective (Damn it, Jim, I'm a developer not a lawyer!), but does anyone find it a bit strange that Microsoft is suing Jamie Cansdale over TestDriven.NET? Jamie's legal troubles stem from TestDriven.NET supporting Visual Studio Express. Microsoft apparently feels that add-ins to VS Express are verboten.* Read Jamie's posts here and here and make your own decision. Is Microsoft really worried that VS Express and TestDriven.NET are such a killer combo that it's going to impact sales of VSTS Developer Edition? My guess is that allowing one add-in opens the door for other vendors to create add-ins. So it's more about establishing precedent than Microsoft feeling that unit testing and code coverage integration are not appropriate for hobbyist developers.

Regardless of Jamie's legal troubles, TestDriven.NET is an excellent add-in. In an effort to support software that makes my development life easier, I've bought a copy of TestDriven.NET. Thanks for all your hard work, Jamie.

* If add-ins aren't allowed in VS Express, does Microsoft have to sue themselves over Popfly Explorer? (I can't take credit for this as it was pointed out by Stephen Oakman, a commenter on Jamie's blog.)

UPDATE: Bil makes a good point that XNA Game Studio Express is another Microsoft-made extension to Visual Studio Express. Personally I don't buy the "Microsoft is giving Express away for free and therefore should be able to extend it if they want" argument. If that was a valid argument, I would recommend to Microsoft that they give away Windows for free. Then Microsoft could tell various third-party software companies (anti-virus, Adobe, Sun, etc.) that their products violate the EULA for the free Windows operating system, but release competing products. Well, they wouldn't actually be competing because there would be nothing to compete with. Note that this would be a completely different story if a product enabled unavailable premium features, such as Vista Ultimate Extras on a non-Ultimate editions. We're talking about extending a piece of software to which you have a license.