I've been a long time users of SysInternals freeware products, in particular ProcesssXP (or Process Explorer), a wonderful replacement of Task Manager (although the two are not really even comparable feature-wise, with ProcessXP providing many more features in all areas). I've used also many of their other tools, like file monitoring and registry monitopring utilities. Fully recommended to any serious Windows developer, not to name the hackers.
Now, as you can read in this press release, Microsoft has acquired SysInternals along with the products-oriented (I mean, not free ones) WinInternals. But the Redmond powerhouse was certainly looking for for the founders expertise than the tools. Mark Russinovich and Bryce Cogswell, along with others, have also produced high quality training classes and videos on Windows Internals and are certainly Windows security experts, something Micrsofot itself seems to lack and to desperately need to ship Vista on time... but that's another story.
Mark is an expert in low-level software, including rootkits. He was the person who found Sony's rootkit used for DRM. You have certainly heard about this story, that hit the news for some time... Hopefully, MS won't use his rootkit expertise to help the DRM movement...
In his blog, Mark claims he'll "be working on challenging projects that span the entire Windows product line and directly influence subsequent generations of the most important operating system on the planet. From security to virtualization to performance to a more manageable application model, there’s no end of interesting areas to explore and innovate." The good think is Mark can certainly help Windows improve and the general avaialbility if his tools (some of which might even end up in the product) could be a good thing.
It is worth noticing that this is a person deeply rooted in Win32/Win64 programming, with a strong dislike for .NET, as you can see in this entry of his blog, titled "The Coming .NET World: I'm scared". Interesting reading, for sure. This is why I fear we are loosing one of the few free voices with a strong technical background on Windows. This could be sad. Hopefully Mark will be able to balance his independence with his new employee...