http://npoi.codeplex.com/.NET version of POI Java project at
http://poi.apache.org/. POI is an open source project which can help you read/write xls, doc, ppt files.
a. generate a Excel report without Microsoft Office suite installed on your server and more efficient than call Microsoft Excel ActiveX at background;
b. extract text from Office documents to help you implement full-text indexing feature (most of time this feature is used to create search engines).
c. extract images from Office documentsd. generate Excel sheets that contains formulas