Have a need? There's an appliance for that.

Virtual appliances can make your life easier by providing turnkey services to your users with a few mouse clicks and a bit of configuration. Since most virtual appliances are free, you have no barriers.

Yes, there's an appliance for that--a Virtual Appliance. Virtual appliances are small, single-purpose virtual machines. You could create them yourself but you probably don't have to, since there are virtual appliance directories hosting hundreds of ready-made, ready-to-use appliances free for the taking. Read the fine print because some of the appliances are trial versions whose creators expect payment after a few day's use.

Virtual appliances have replaced physical appliances for many vendors. A few years ago, network appliances were booming but now almost all are gone in favor of their more efficient and less costly virtual cousins.

VMware alone hosts hundreds of Virtual Appliances (VAs) in their Appliance Marketplace. Parallels hosts about 100 VAs in their Virtual Appliances Directory. ThoughtPolice offers dozens of Linux VAs that cover every major Linux distribution plus some FreeBSD VAs. And, finally, JumpBox.com creates and hosts a gaggle of appliances in their Virtual Appliance Library.

There's a virtual appliance available to serve almost every imaginable need or request. From simple shell access to web application services, chances are good that if you've thought of it, someone else has too and he's created an appliance for it. Virtual appliances come in two basic categories: free and commercial. And, just so you know, you can find some pretty sophisticated free appliances that are every bit as good as commercial ones. The primary difference between free and commercial is priority vendor support. Many so-called commercial appliances are available free of charge unless you need support. If you can support them yourself, they're totally free.

The best part of VAs is that they come to you pre-configured and ready to serve. Additionally, they are easy to customize for your environment via setup scripts or web interfaces. Some appliance creators provide their own repositories from which your updates, patches and fixes come.

OK, so there's a small catch with virtual appliances. They're mostly made with VMware in mind as the host. The upside is that other virtualization software solutions either can convert vmdk files or use them natively, so you aren't locked in to a VMware-only solution. Some clever virtual appliance creators supply their appliances in OVF format so that they're easily imported into the leading hypervisors.

Do you use virtual appliances in your virtual data centers? Write back and let me know which ones are most useful to you. I'm especially interesting in knowing if you've created any yourself.