There is only one file type that uses the .NUSPEC extension:
NuGet Specification File
File used by NuGet, a Microsoft Visual Studio extension used for managing third-party libraries for .NET projects; saved in an XML format and contains information that is used to build a NuGet package (.NUPKG file), which subsequently can be added as a Visual Studio project library.
NUSPEC files are often created from C# projects (.CSPROJ files).
Updated: June 17, 2012