If you want the executable:
System.Reflection.Assembly.GetEntryAssembly().Location
If you want the assembly that's consuming your library (which could be the same assembly as above, if your code is called directly from a class within your executable):
System.Reflection.Assembly.GetCallingAssembly().Location
If you'd like just the filename and not the path, use:
Path.GetFileName(System.Reflection.Assembly.GetEntryAssembly().Location)