C programming language offers 25 standard header files that are supported by default. Some important standard header files include:

1. #include <stdio.h> (Standard input-output header)

2. #include <string.h> (String header)

3. #include <conio.h> (Console input-output header)

4. #include <stdlib.h> (Standard library header)

5. #include <math.h> (Math header)

6. #include <ctype.h> (Character type header)

7. #include <time.h> (Time header)

8. #include <assert.h> (Assertion header)

9. #include <locale.h> (Localization header)

10. #include <signal.h> (Signal header)