I've spent some time today trying to find .NET libraries and tools to explore mathematics and I have to report that it seems like slim pickings!
There are a few that cost money, but hey, I'm living off a developer's salary so I can't afford to buy software. Wish I could. But, this is 2011 and I'm afraid to spend money like everyone else.
As far as open source goes...
Math.NET I had a hard time finding documentation or samples using this project that helped me see how I could use it for exploration of calculus. I did figure out how to get up and running with some combinatorics examples but right now I'm reading calculus. Maybe its better for other topics other than calc?
Smart Math Library The documentation looks pretty extensive but I haven't made much time to explore.
Microsoft Graphics calculator sample
I think this sample is to teach WPF but I will take a look at it and see how they coded up a fairly basic graphics calculator.
Looks like there is a lot of work and opportunity in this area.