Finding SharePoint GUIDs using PowerShell

2010 version:

$site = Get-SPSite http://yourserver/sites/yoursite
$web = $site.OpenWeb("yoursubsite")
write-host "Site: " + $site.id
write-host "Web: " + $web.id
$web.lists | Format-Table title,id -AutoSize
$web.Dispose()
$site.Dispose()

Note: You could change $site.OpenWeb("yoursubsite")  to $site.RootWeb to get just the top site.
 

Here’s what the output looks like: