.NET Micro Framework
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This is what I was expecting Silverlight would be based on, not the CoreCLR.
So that gives us desktop CLR 1.0, 1.1, 2.0 and 2.0 for Vista (not that you'd notice by looking at the version numbers, but 2.0 on Vista has changes e.g for ASP.NET to support IIS7). .net 3 doesn't touch the CLR, but I'm not sure about 3.5.
The .net Compact Framework has gone through a couple of versions, and even has a subset for the Xbox 360. I've no idea how many versions of the .net Micro Framework there are (does this power those SPOT watches?) And now we've got the CoreCLR, which is cross platform. Oh, and Rotor and Mono, too.
Not to mention all the various service packs.
That's an awful lot of .net.
All the more reason for Microsoft to ensure that the CoreCLR is a true subset (I'm talking common code base), and is strictly compatible with the desktop CLR. I'd hate to have to work up a compatibility matrix for that lot.