Right now VMware has the most installations. With Microsoft licensing scheme now clearly defined in virtual environments expect Hyper-V to become a big player as well, they easily beat VMware on price. With that said, VMware still has the most features and is the most mature of all them. If I am an enterprise, VMware is the way I would go. In up front costs Hyper-V is the clear winner. But when taking into account when things go wrong or maintenance I would go with VMware.
For those few that need raw performance Xen is the way to go.
For the Mac I prefer Fusion to Parallels.
We are using VMware and Hyper-V. As a training company we are getting a lot of interest in Hyper-V - way more than even a month ago.. VMware is by far the biggest in the market but Hyper-V is gaining ground fast. They both work well and are equal as far as administration is concern. VMware can scale further today but how many virtual machines do you really need? I've spoken to some of the largest VMware shops in Hawaii and they are taking a hard took at Hyper-V because of the total cost.
Internally Hyper-V is solid - we've had Zero problems. We constantly spinning up new servers for all of our administration, SQL and SharePoint classes. The stuff just works. As for our production servers, everything is being migrated to Hyper-V. It is scaling well.
Slick! I hadn't thought of running virtual machines on my phone. I suppose it's possible.