Does anyone know of a good open-source zipping library for .NET?
Regarding the comments and other posts about the internal gzip implementation, they are not the same! GZip does not create the header required for archiving; it is only useful for "zipping" one file or stream.
Proper zip archives contain a header that list all compressed files and where in the compressed data they come and therefore you need something that makes a header. That means SharpZipLib, one of the many commercial versions or using something external with .NET bindings like 7zip.
Just on the offchance somebody wants to say this: "But I see .gz files in Linux all the time!" - they're just single files and .tar.gz is no exception - tar is the archive file. The .gz is that archive compressed.