Features:
• Without ads
• Puzzles can be downloaded from web entered by hand or generated on the project's homepage
• Game stats (time spent mistakes)
• Next step hints with multiple strategies suggestions
• Import to and export from a file
• Visual themes including custom theme editor
• Several input modes (finger-friendly numpad popup keyboard etc.)
• Double pencil marks
…and much more.

More information (including how to report ideas and issues) can be found on the project's home page: http://opensudoku.moire.org