How to get the Serial Number from an ESXi Host

Directly on the ESXi host:

esxcfg-info | grep -w "Serial Number"

Remotely with PowerCLI:

$myCredentialsObject = Get-Credential
Connect-VIServer -Server <ServerName> -Credential $myCredentialsObject
Get-VMHost |
Select @{N='VMHost';E={$_.Name}},
    @{N='Serial';E={($_.ExtensionData.Hardware.SystemInfo.OtherIdentifyingInfo |
    where {$_.IdentifierType.Key -eq "ServiceTag"}).IdentifierValue}}