I use Virtual Machines a lot. I have used VMware, Virtualbox, Parallels, and Hyper V. My personal favorite is Hyper V if you are in a Windows domain environment. It doesn't have all of the features others have, but for me it works great.
I use it at work. I am currently in the middle of transitioning everything over to Hyper V. The last Sysadmin was a fan of VMware, and ESXi, but he never actually installed any keys in the ESXi servers so their functionality is limited. I will admit, that VMware has some very cool, advanced features, but you have to pay for them.