FlexCel Studio is a suite of native .NET components allowing to read, create and modify native Excel (xls) files, without needing to have Excel installed. Supported Excel 97 and up.
Completely written on C# 100% managed code, with no interop and no p/invokes.
It has an API that can write and read files, with basic formatting info, images comments, etc.
While creating reports, you have more options available and you can use images/comments/conditional formats/merged cells/pivot tables/charts and almost the complete Excel 2000 feature set.
A Report document is included to natively see the generated reports.