Download it on CodePlex here.
CCI can read/write metadata contained inside .NET assemblies. My guess is that CCI got developed by Microsoft to get a better System.Reflection. Indeed, I explained previously in the post Mono.Cecil vs. System.Reflection that System.Reflection comes with numerous flaws. Btw, CCI and Mono.Cecil are 2 framework with the exact same goal. Static analyzer like FxCop are based on CCI while NDepend is based on Mono.Cecil.
From my point of view the real revolution is that by releasing CCI OSS, Microsoft exposes openly the internal of PDB files, (the files used by the debugger to link code source and binary code). As far as I know PDB has been always a closed format (although several unmanaged PDB reader framework are available). And as explained here, PDB format doesn’t look like a simple flat file format. This is a very good news for all .NET developer tools ISV provider because PDB files contains invaluable information and we can now have our hands in it!