SharePoint 2010 - Enabling the Developer Dashboard

Enabling via powershell:

$contentService = [Microsoft.SharePoint.Administration.SPWebService]::ContentService
$dashboardSetting = $contentService.DeveloperDashboardSettings
$dashboardSetting.DisplayLevel = [Microsoft.SharePoint.Administration.SPDeveloperDashboardLevel]::On
$dashboardSetting.Update()

http://msdn.microsoft.com/en-us/library/office/gg512103(v=office.14).aspx
http://msdn.microsoft.com/en-us/library/office/ff512745(v=office.14).aspx