ClosedXML is a .NET library for reading, manipulating and writing Excel 2007+ (.xlsx, .xlsm) files. It aims to provide an intuitive and user-friendly interface to dealing with the underlying OpenXML API.
ClosedXML.Report is a tool for report generation with which you can easily export any data from your .NET classes to Excel using a XLSX-template.
Wiki for ClosedXML
MVC Extensions for ClosedXML
ASP.NET extensions for ClosedXML
Parse and render Excel number format strings