In the hacking mini-game, the login password "COWBOYCURTIS" will unlock the same messages from Morpheus as the password "FREEMIND". The actor who played Morpheus, Lawrence Fishburn, also played a character called Cowboy Curtis in Pee Wee's Playhouse.
There's a hidden message from Neo hidden deep within the Hacking section of the game. Following a long process which unlocks the majority of the hidden content, the player will receive a voiced message from Neo saying "You possess great skills. Now it's your turn. Follow the white rabbit."