http:
1xx Informational
+ 100: Continue (:continue)
+ 101: Switching Protocols (:switching_protocols)
+ 102: Processing (:processing)
+ 103: resume aborted PUT or POST
+ 122: URI is longer than a maximum of 2083 characters
2xx Success
+ 200: OK (:ok)
+ 201: Created (:created)
+ 202: Accepted (:accepted)
+ 203: Non-Authoritative Information (:non_authoritative_information)
+ 204: No Content (:no_content)
+ 205: Reset Content (:reset_content)
+ 206: Partial Content (:partial_content)
+ 207: Multi-Status (:multi_status)
+ 226: IM Used (:im_used)
3xx Redirection
+ 300: Multiple Choices (:multiple_choices)
+ 301: Moved Permanently (:moved_permanently)
+ 302: Moved Temporarily (HTTP/1.0)
+ 302: Found (HTTP/1.1) (:found)
+ 303: See Other (HTTP/1.1) (:see_other)
+ 304: Not Modified (:not_modified)
+ 305: Use Proxy (:use_proxy)
+ 306: (no longer used, but reserved)
+ 307: Temporary Redirect (:temporary_redirect)
4xx Client Error
+ 400: Bad Request (:bad_request)
+ 401: Unauthorized (:unauthorized)
+ 402: Payment Required (:payment_required)
+ 403: Forbidden (:forbidden)
+ 404: Not Found (:not_found)
+ 405: Method Not Allowed (:method_not_allowed)
+ 406: Not Acceptable (:not_acceptable)
+ 407: Proxy Authentication Required (:proxy_authentication_required)
+ 408: Request Timeout (:request_timeout)
+ 409: Conflict (:conflict)
+ 410: Gone (:gone)
+ 411: Length Required (:length_required)
+ 412: Precondition Failed (:precondition_failed)
+ 413: Request Entity Too Large (:request_entity_too_large)
+ 414: Request-URI Too Long (:request_uri_too_long)
+ 415: Unsupported Media Type (:unsupported_media_type)
+ 416: Requested Range Not Satisfiable (:requested_range_not_satisfiable)
+ 417: Expectation Failed (:expectation_failed)
+ 422: Unprocessable Entity (:unprocessable_entity)
+ 423: Locked (:locked)
+ 424: Failed Dependency (:failed_dependency)
+ 426: Upgrade Required (:upgrade_required)
5xx Server Error
+ 500: Internal Server Error (:internal_server_error)
+ 501: Not Implemented (:not_implemented)
+ 502: Bad Gateway (:bad_gateway)
+ 503: Service Unavailable (:service_unavailable)
+ 504: Gateway Timeout (:gateway_timeout)
+ 505: HTTP Version Not Supported (:http_version_not_supported)
+ 507: Insufficient Storage (:insufficient_storage)
+ 510: Not Extended (:not_extended)
Further info (w3 spec):
http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html