Puzzles generated by gnome-sudoku
gs4 easy	games: 100
gs4 medium	games: 100
gs4 hard	games: 100
gs4 very_hard	games: 100
gs5 easy	games: 100
gs5 medium	games: 100
gs5 hard	games: 100
gs5 very_hard	games: 100
gs6 easy	games: 100
gs6 medium	games: 100
gs6 hard	games: 100
gs6 very_hard	games: 100
Puzzles from www.sudocue.net
The No-Trick Ponies Collection	games: 250
The Learning Curve Collection	games: 2500
The Superiors Collection	games: 160
The Almost Daily Nightmare Collection	games: 400
100 Snowflake Puzzles	games: 100
Top 10000 Sudokus	games: 10000 :-)))) source