There are no agents for ESXi because it hasn't got a service console. Your best bet would be scripting this and kicking of the script from another physical machine when power fails.

Duncan
My virtualisation blog:
http://www.yellow-bricks.com

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Please reply to me if you have questions, I accidentally posted that from another account.

I solved the problem with a Web/SNMP network management card connected via IP to a VM box.
Then I compiled the apcupsd driver (http://www.apcupsd.org) inside a Linux VM box and in case of a
power failure I send the /sbin/poweroff command to the ESXi host via ssh using an identity file.

The trick is to enable the SSH server inside the ESXi host, build a dropbear and openSSH RSA
root key with at least 768 bit on it, put all the keys inside the oem.tgz file, disable root login and
for safety reasons enable only the execution of remote commands (append -g to ssh line inside
inetd.conf) and configure the shutdown and startup sequence of every VM boxes.

Beforehand I tried the PERL Toolkit and stumpled over the problem that sending a shutdown command
using the vmcontrol.pl command prevents the ESXi host to shutdown itself properly.

Please consider to use a big UPS in case you have plenty VM boxes to shutdown and don't forget to plan
the startup and shutdown sequence of all involved VM boxes with care.

I understand enabling SSH on ESXi works, but it's not supported by VMware (and it could be disabled in the future, no one knows). VMware itself tells you to use the remote CLI tools for this.. You must have a good reason to persist the ssh solution

This isn't supported but here you go. I used puttykeygen to create the key pair.

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1) Generate the public / private key pair
2) Edit /etc/passwd - add "root" to the root entry so the line will be like this
root:x:0:0:Administrator:/root:/bin/ash
3) Run mkdir -p /root/.ssh
4) Copy the public key to /root/.ssh/authorized_keys
5) Run chmod -R 700 /root
Attempt to login with your SSH client.

The bundle the /root folder into oem.tgz. You could also do /etc/passwd and /etc/inetd.conf, but I only tested the /root folder.

Puttykeygen generated both a public key file and text to paste into authorized_keys. I used the text part (the public key file was formatted differently) so I end up with this in the file on ESXi.

ssh-rsa AAAAB3NzaC1yc2EAAAABJQAAAIEAor/Mm2qGk+H+ZRNbjx/3e0Qz2cNEKEbzsFiYTO3znvwfWqJjzb7sD1bue9VnyyUEBu5ylMs0md9xZpZ/U6HIKwoDwckZQr4/48sHv7B3dU6I4pyD5H7bvpvnfK2Qh5/VCad5aGGKvdDrDzIv4K/l1uPso9PNIpzch9lEuD7il0k= rsa-key-20081106