VirtualBox is a virtual emulator like VMWare workstation. It has many of the features VMWare has, as well as some of its own.
I really like new Opensource VirtualBox from Sun. It is light on resources. Here is a quick tip - you can convert a VMware virtual machine to a VirtualBox machine using qemu-img utility.
qemu-img is included with qemu package. You can install it by typing:
$ sudo apt-get install qemu
Convert a VMWare Image to VirtualBox Image
Convert VMWare image called centos.vmdk to /tmp/centos.bin
$ qemu-img convert centos.vmdk /tmp/centos.bin
Now use VBoxManage to get back image in native format:
$ VBoxManage convertdd /tmp/centos.bin centos.vdi
I hope this quick tip will save some time.