Most open source C/C++, Java, Python, Ruby, OCaml, and other libraries can be compiled against MSIL and called natively from C# or consumed as binaries and invoked via common C calling conventions.

As for the IDE dependency complaint, I think this applies more towards some of the standard libraries such as WPF, WinForms, ASPX, ADO.net, etc. I think it is a very valid complaint, but I don't think it is about the language itself.