Powershell makes it easier to download a WSP from SharePoint 2010 as there is no option available in Sharepoint 2010 central administration. Even with stsadm, there is no command to download the wsp.

The following Powershell code will help extract all the wsp files from current Sharepoint farm.

$dirName = ""
Write-Host Exporting solutions to $dirName
foreach ($solution in Get-SPSolution)
 $id = $Solution.SolutionID    
 $title = $Solution.Name    
 $filename = $Solution.SolutionFile.Name    
 Write-Host "Exporting ‘$title’ to …\$filename" -nonewline     
    Write-Host " – done" -foreground green    
 } catch    
    Write-Host " – error : $_" -foreground red