A simple script shared at @ http://secretsofsharepoint.com/cs/ site for Checking in the documents in a document library using a powershell script.
function CheckInDocument([string]$url)
{
$spWeb = Get-SPWeb $url
$getFolder = $spWeb.GetFolder("Shared Documents")
$getFolder.Files | Where { $_.CheckOutStatus -ne "None" } | ForEach
{
Write-Host "$($_.Name) is Checked out To: $($_.CheckedOutBy)"
$_.CheckIn("Checked In By Administrator")
Write-Host "$($_.Name) Checked In" -ForeGroundColor Green
}
$spWeb.Dispose()
}
Here's an example on running the function:
CheckInDocument http://SP