LevelSets:

This are a few sites where Sokoban level authors have put their levels on-line.

Puzzles by David W. Skinner
Puzzles by Evgeniy Grigoriev (GRIGoRusha)
Puzzles by Aymeric du Peloux
Puzzles by François Marques
Puzzles by Yoshio Murase
Puzzles by Lee Haywood
Puzzles by Kenya Maruyama (kenyam)
Puzzles by Erim Sever
Puzzles by Zbigniew Kornas (ZIKO) (or also at the SokoFun website)