Vexed
Vexed, the classic Palm game made in C# for .NET 4
...It's one of the best puzzle games I've ever played and decided it needed a makeover for Windows.
Open source, of course, you'll find the original on Sourceforge still at http://vexed.sourceforge.net/.
I hope you enjoy, but remember, it's not called 'Tough' or 'Difficult', it's called 'Vexed'. I will not be responsible for a lack of sleep on any players part.