Xen, It is used by some of the biggest clouds in the world and it has a lot of new development happening as well.

+1

Especially since XenServer is Open Source, allowing fast deployments with a nice api (XAPI). For an example of possibilities, go check Xen Orchestra project :)

Xen (yes, I'm joining the party :) ). Paravirtualization works great and with PVH coming out shortly, one gets the best of both worlds by using hardware virtualization where it's most effective.

Also, Mirage OS is a cool new use of Xen and is one of the first things I've seen that really does deserve the use of a term like "cloud-based" to describe it as it's not just throwing another OS in a VM just to run one or two applications. Since Xen powers the big public clouds (eg. AWS) you can use it there right now.

Virtualbox (due to the simple installation and VM management)

Meeting VBox was one the best things that happened to my virtual life!

Preferred virtual system
 

I use XEN and KVM.

Xen
 

I work with Xen and Xenserver open source. In my case the best solution after probe many others virtualization system.

VirtualBox

It's tough to say:
I only use VirtualBox and it does what I want (allowing me to play with other distros and OSs quickly and easily. However, it does use a noticeable amount of processing power to do so.
In my experience on my system VMWare seemed to be worse, but I understand this is not usually the case.
I should use KVM and the rest but for me as a home hobby user they're too much like hard work to get up and running.
Now I know about LXC I'll have a look at that though as if it's along the lines I get the impression it is it could be great for running two Linux distros at the same time at native speed which could be ideal for a few things I'd like to do.
Can I vote for them all?!

At work I use VMWare ESXi and works great emulating Windows, CentOS, Ubuntu Server, ... As a Desktop app I use VMware Fusion (for Mac) and Parallels Desktop (also for mac) and all two work great, perhaps VMWare needs less CPU and GPU.
I vote VMWare.

I do think vmware is good. They have been at it for a while. Just not sure i like their licensing.

I like virtual box because it has usb support pre made options for os' and just plain works

Qemu hands down. Virtualbox has issues on pure x86_64

I like QEMU with KVM.

Shouldn't ovirt (the FOSS upstream of Red Hat's RHEVM product) be mentioned?

http://www.ovirt.org