We have been using an open source library called ExcelPackage at work for generating spreadsheets based on some data from a database. I really like the library, the API is nice and easy and allows for some powerful stuff. There are some things that I would like added (and some other people have things they want added), but the developer seems to have disappeared.
I decided to fork the project and so have created the ExML project on CodePlex. I was originally thinking of calling it ButterCover (I was tired when I came up with it), but changed to ExML later.
Please put requests in the discussions are on the project and I'll try and implement them as quickly as I can.