It has Yoshi in it, F***ING YOSHI IN SMB3! Plus lots of other stuff from other Mario games ported in!
"Super Mario Bros. 3Mix is a large-scale hack that changes and adds lots of features to Super Mario Bros. 3, and pays homage to many past and (relative) FUTURE Mario titles, while attempting to remain faithful in spirit to the look and feel of Super Mario Bros. 3." Website