ClosedXML - The easy way to OpenXML

https://github.com/ClosedXML/ClosedXML
closedxml.zip (108,9 MB)
ClosedXML makes it easier for developers to create Excel 2007+ files.

NPOI

https://github.com/tonyqus/npoi
npoi.zip (120,3 MB)
.net port of Apache POI

PHPExcel (OLD DEAD REPOSITORY) - visit PHPOffice/PHPExcel on github instead

https://github.com/PHPOffice/PHPExcel
PHPExcel.zip (126,9 MB)
PHPExcel (OLD DEAD REPOSITORY) - visit PHPOffice/PHPExcel on github instead

Simple OOXML

simpleOOXML.zip (769 kB)
Simple OOXML makes the creation of Open Office XML documents easier for developers. Modify or create any .docx or .xlsx document without Microsoft Word or Microsoft Excel. Uses the Open Office SDK v 2.0.

Tambourine.NUnit

TambourineNUnit.zip (1,7 MB)
Provides pivot report creating from NUnit combinatorial test result xml files. Also supports viewing of pivot data and its exporting to xlsx.