I like VirtualBox
It has a very friendly user interface and in general works pretty well.
If I had to find fault it, the speed of the guest / host virtual file system is horrible which makes it very hard to contemplate using it with something like Vagrant to virtualize development environments. I packaged up a compiler and some other tools into a vagrant box and while the compiler ran at about 90% of native speed (not bad) locally, it took close to 10x longer to compile files out on the host fs due to the virtualization.
It would also be nice if operating systems came with a standard set of hooks so there wasn't a constant need to install guest drivers