I always forget the name of the class that stores a great big list of field ID constants. So as a reminder to myself it’s SPBuildInFieldId
For example:
SPFieldUserValue user = item[SPBuiltInFieldId.Author]
Whilst looking this up again I stumbled across another similar class SPBuiltInContentTypeId that holds content type id’s for the common OOTB content types, neat!
if (this.ContentTypeId.IsChildOf(SPBuiltInContentTypeId.Message)) { ...... }