Built in clustering & high availability!

Even if you have an application which is CPU hungry (say Exchange) you can drop it into an ESX farm and allocate resources equivalent to a single physical server - then if you leverage VMotion and High Availability if the Exchange server crashes you can migrate it (manually or automatically) to another physical server with minimal downtime. In the old-days if the server failed catastrophically you potentially needed to re-install and restore from backup (assuming you had spare hardware available).

From here Changing the IP address of service console in ESX 3.x :

esxcfg-vswif -a vswif0 -p Service\ Console -i 10.1.1.1 -n 255.255.255.0 -b 10.1.1.255

And don't forget to set the correct gateway in /etc/sysconfig/network or the command to configure the virtual switch interface will hang.

ESX is very cool and they've made it pretty compelling in terms of a step up on the free Server (and older GSX) versions. It include user ACL, virtual switching, more efficient hypervisor (the RedHat 7.2 upon which ESX is based is stripped to the bare bones) and more granularity in terms of resource allocation. One of the things that isn't made very clear is that if you want to leverage some of the bells & whistles (eg High Availability, VMotion, Backup, centralised licensing) you'll need a SAN (or NAS in a pinch) and another box - ideally physical although it could be virtual (obviously you can't do HA or VMotion if your ESX instance hosting the management box dies though!).

Two products I came across while reading a previously linked article offering datacenter tips - PlateSpin PowerConvert and Leostream.

The fantastic thing about these products is that they let you seamlessly migrate physical servers into virtual servers.

This is amazing stuff:

Stream servers between physical servers, blade infrastructures, virtual machines, and image archives over the network

Gone are the bad old days of Systems Admin where you needed to audit and plan every aspect of a server migration to new hardware. Even then it was so easy to miss something - particularly with Windows which doesn't offer much in the way of storing a running config - sure you can restore a registry but if you try that to different hardware you'll render the target system inoperable. Now the only risk is a virtual machine going 'stale' as it gets moved from one virtual environment to another. At least with a hardware refresh you're forced to look at what services can be dropped or how an install can be improved when starting with a clean slate.

If you didn't already know the good folks have released VMWare Server as freeware (you do need to register though).