I know libtiff for C, but haven't found a port for .NET
update
I found FreeImage, which is based on several different libraries and provides a .NET wrapper.
What about using the built in .NET class? See "How to: Encode and Decode a TIFF Image" on MSDN.
I'm sorry to resurrect this old thread, but my company recently released a free and open-source version of LibTiff written using only managed C# (license permits commercial use and distribution).
http://bitmiracle.com/libtiff/
I hope it will be useful for you or others.