devCompress.Net - A Zip compression library for .NET developers. - http://www.devbiz.com/devcompress/
Chilkat ZIP.NET - Implements standard Zip compression tasks. Zip and UnZip files and directories, or directly from application memory. - http://www.chilkatsoft.com/dotNetZip.asp
Xceed Zip for .NET - Create and manipulate industry standard zip files and perform other data compression tasks. - http://www.xceedsoft.com/products/ZipNet/