How to create a zip archive with PowerShell?

A pure PowerShell alternative that works with PowerShell 3 and .NET 4.5 (if you can use it):

function ZipFiles( $zipfilename, $sourcedir )
{
   Add-Type -Assembly System.IO.Compression.FileSystem
   $compressionLevel = [System.IO.Compression.CompressionLevel]::Optimal
   [System.IO.Compression.ZipFile]::CreateFromDirectory($sourcedir,
        $zipfilename, $compressionLevel, $false)
}

Just pass in the full path to the zip archive you would like to create and the full path to the directory containing the files you would like to zip.

PowerShell v5.0 adds Compress-Archive and Expand-Archive cmdlets. The linked pages have full examples, but the gist of it is:

# Create a zip file with the contents of C:\Stuff\
Compress-Archive -Path C:\Stuff -DestinationPath archive.zip

# Add more files to the zip file
# (Existing files in the zip file with the same name are replaced)
Compress-Archive -Path C:\OtherStuff\*.txt -Update -DestinationPath archive.zip

# Extract the zip file to C:\Destination\
Expand-Archive -Path archive.zip -DestinationPath C:\Destination