ExcelPackage is open-source software that provides server-side generation of Microsoft Excel 2007 spreadsheets. It is a set of classes and wrappers around the .NET 3.0 System.IO.Packaging API and the new Office Open XML file format. It abstracts away the complexity of dealing with the individual XML components making it very easy to create sophisticated spreadsheets on the server. Microsoft Office need not be installed.
A more recent effort to establish a similar functionality was taken on by the project EPPlus that is also maintained on codeplex and was based on ExcelPackage.