So maybe I will even make a series of "Hard to find HP tools" posts. Anyway I will start with the Offline ACU tool today.
So, what do you need this tool for? I had this challenge before and I reminded that when I came across this VMware Community forums post: Imagine you have an HP based ESXi host with VMs running on local disks attached to a Smart Array RAID Controller. You have run out of disk space and decide to add an additional hard disk to the server. Instead of creating a new (unprotected) RAID volume on this single disk you prefer to expand an existing RAID volume with it. This will give you more disk space and keep the current RAID protection level. How do you do that?
No problem, if you had Windows (or Linux) running directly on the box, because HP made available the Array Configuration Utility (ACU) for these operating systems. It will allow you to do the RAID expansion online while the OS is running.
Update (2013-06-04): Please note that HP also made available hpacucli for ESXi meanwhile. It is included in the HP Offline Bundles and their customized installation images. See this post and my HP&VMware links page.
You can still
You can find the download link to the current version of the HP ProLiant Offline Array Configuration Utility on my HP & VMware links page (in the General section).
Once you have successfully expanded your RAID volume (and booted into ESXi again) you just need to do the same with the VMFS datastore that resides on it. Please note that since vSphere 4.0 you can grow a VMFS datastore online, and you do not need to use VMFS extents. Choose "Increase..." from the datastore's properties menu: