* * "Everything Short of an Engine!" * *
Correspondence Chess allows the in-game use of databases, books or any outside help (not chess engines or other players) - while playing.
This discussion http://www.chess.com/forum/view/general/a-tally-of-database-users--non-users proposed a experimental match between Users & Non-Users.
(See also http://www.chess.com/forum/view/general/using-books--databases-for-playing-turn-based)