GitHub repository: JDeuker/ClosedXML
Primary language: C#
Description: ClosedXML makes it easier for developers to create Excel 2007+ (.xlsx, .xlsm, etc) files. It provides a nice object oriented way to manipulate the files (similar to VBA) without dealing with the hassles of XML Documents. It can be used by any .NET language like C# and VisualBasic.NET.