How to log in to an ESX host's service console

If you have access to the host's physical console (e.g. by KVM or HP iLO) then press Alt-F1 there to get a username/password prompt and log in as user root.

If you want to access the service console remotely (via ssh), follow these instructions:
  Enabling root SSH login on an ESX host
  Connecting to an ESX host using a SSH client

How to log in to an ESXi host

ESXi has no service console, but it still has a shell running as a so-called userworld of the VMkernel. To access it locally through the host's physical console  (e.g. by KVM or HP iLO)  follow these instructions:

  For ESXi 3.5 and 4.0:  Tech Support Mode for Emergency Support
  For ESXi 4.1: Using Tech Support Mode in ESXi 4.1

The article for ESXi 4.1 also explains how you can do this remotely (via ssh). This is not supported for ESXi 3.5 and 4.0. Although there is way to get this working also: see here.

How to edit a VM’s advanced settings without powering it off

In the vSphere client you can edit a VM’s advanced settings only while it is powered off. However, with a PowerCLI script you can do that on a running VM. And to make the change effective you just need to vMotion the VM to another host!

Here is a PowerCLI script sample:

Connect-VIServer name-of-vcenter-server-or-esx-host
$vm = Get-View (Get-VM NameofVMtoUpdate).ID
$CfgSpec = New-Object VMware.Vim.VirtualMachineConfigSpec
$CfgSpec.extraconfig = New-Object VMware.Vim.optionvalue
# you can use any advanced setting key/value pair here:

If you haven't used the PowerCLI before: It's worth learning it ... You should at least have an up-to-date installation of it somewhere to be ready to run such useful scripts like this one.

Visit VMware's PowerCLI page to learn more!