I have updated DotNetZip, the open source Zip Library for .NET, to v1.3. 

[Update, 3 January 2009:  DotNetZip is now at v1.7 .  It includes Unicode, ZIP64, and runs on the Compact Framework. ]

Find it at http://www.codeplex.com/DotNetZip .

Some of the key fixes new features include: ability to read and write zips via streams, better interop with Java+Linux, better support for ASPNET (temporary file folder), support for comments in the zipfile and on each zip entry, support for overwrite.  There are a bunch of other fixes too.  

If you want some background on the Zip Library, check out my prior posts on this topic.  Just for your information, DotNetZip is licensed under the Microsoft Permissive Public License, which basically means you can use it free of charge, for any purpose, but there's no warranty.  Check the full terms of the license on the CodePlex site.

As always if you have questions on usage, check the CodePlex forums.