Huh, I never knew this. For anyone else who didn't know, it's in the C:\Windows\Microsoft.NET\Framework64\ folders, it's the CSC.exe file included with each version of the .NET framework.
Edit: Good grief, there's even vbc.exe, which is a command line Visual Basic 2012 compiler. Also JSC.exe for Jscript, aspnet_compiler.exe for ASP and MSBuild. Kinda embarrassed that I didn't know this.