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I stumbled across some information on "Sandcastle" this morning. Sandcastle creates documentation for any managed class library. There is a lot of good information on the Sandcastle teams blog. For internal documentation, or for documenting your APIs that you are exposing to allow your customers to integrate with your software I think you will find this an interesting product.
The latest release is the March CTP which you can download here. And you can read the announcment around the March CTP here, including a list of What's New, Changes & Fixes.