You should first ask yourself what you want to virtualize. Actually... you first need to clarify is it business or home (small office maybe) use.
There are many options, just to name a few:
- hardware consolidation (most common reason)
- lab/test/edu environment setup (another common thing even though I agree with Blueline here)
- ease of management/administration (many many examples)
- disaster recovery/high availability
- desktop consolidation/thin clients (Citrix, Terminal Services...)
- infrastructure consolidation (Infiniband, SAN in general...)
I am a bit rusty on the subject and only know couple of products (no knowledge on Xen, M$, Apple desktop stuff,...) but I won't be far off if I say that only VMware delivers in x86 arena.
I'm VMware 2.5.X certified myself. Not much field experience but I am maintaining couple of servers at work but as said, due to other commitments I have hard time keeping up with the progress. Virtualization is such a buzz word lately covering lots of solutions as you can see from the examples above. It is nothing new though (IBM mainframe, large *NIX servers...) it's just that it's now also playing a role in other areas such as appliances, infrastructure, x86 architecture...
Nice article that might help you start -
Wikipedia