If you are coming to this series of posts for the first time you might check out my introductory post for a little context.
Have you ever wanted to generate a number that could potentially have millions of digits and do arbitrary mathematical operations against it? Ya, me neither. But if I did, I would take a look at this project.
an arbitrary precision integers class written in pure C# with fast - about O(N * log N) - multiplication/division algorithms implementation. Runs on .NET 2.0+.