Categorias:

1xx - Informational
2xx - Success
3xx - Redirection
4xx - Client Error
5xx - Server Error

Alguns status code muito utilizados:

101 - Switching Protocols
200 - OK
201 - Created
204 - No Content
301 - Moved Permanently
302 - Found
400 - Bad Request
401 - Unauthorized
403 - Forbidden
404 - Not Found
405 - Method Not Allowed
409 - Conflict
422 - Unprocessable Entity
500 - Internal Server Error
501 - Not Implemented
502 - Bad Gateway
503 - Service Unavailable
504 - Gateway Timeout