Get whitespace in DB’s:

2010:

Get-MailboxDatabase -Status | Select Servername, Name, AvailableNewMailboxSpace
 
2007:
Get-MailboxServer | Get-MailboxDatabase | Select Server, StorageGroupName, Name, @{Name="Size";expression={"{0:N2}" -f ((get-mailboxstatistics -database $_.Identity | Measure-Object -Property TotalItemSize,TotalDeletedItemSize -Sum |Select-Object Sum |Measure-Object -Property Sum -Sum).Sum.ToString() /1mb)}}