This is a game I am currently developing.
It is created with C# MVC and deployed on Windows Azure.
The game can generate and solve puzzles and gives hints.
The player can enter numbers on the cells by using number buttons.
The player can check for errors whenever they want. The first error is highlighted.
There is an undo button.
It currently cannot hint for all strategies. When the simpler strategies are exhausted it resorts to brute force search to solve puzzles.
Working on: Showing pencil marks. Better hints/coaching for advanced strategies.