VirtualBox – For casual usage, I've come to rely on VirtualBox as it allows me to run virtual machines for Windows or various Linux distributions on my Ubuntu host system quickly and easily. VirtualBox boasts many of the same features as VMware, such as USB device pass-through and simple-to-setup virtualization tools. But one area where VirtualBox differs is with its fantastic, easy-to-navigate user interface. VirtualBox is available both in an open source edition and a release with proprietary components.
Next up, I'll share the virtual machine applications that I've been less impressed with. Your own experiences may vary with these virtual machine software titles; however, I've found them to be lacking for regular usage.
Parallels Workstation – I've never been very impressed with Parallels when using Ubuntu as the host operating system. It literally does nothing better or different than the two options above. Worse, Parallels is also proprietary software and requires registration. If Parallels Workstation appeals to you, consider VirtualBox instead. Based on my own experience, the only differences between Parallels Workstation and VirtualBox is the price.
Qemu – I'm fairly sure I will get a lot of grief for claiming that there is anything wrong with Qemu, considering how customizable and powerful this virtual machine application actually is. But putting aside those positive factors offered by Qemu, the fact of the matter is that it's really not that great with resource usage. Even idling, it's a bit heavy on resources. As for using Qemu, I also think it's a bit too complex for most casual users. I'd happily recommend it to those who wish to study its documentation; however, for anyone else, I would suggest VMware instead.
WINE – I know, WINE isn't virtual machine software. And WINE also isn't an emulator, either. But considering that WINE is often mentioned in articles like this, I thought I'd share my thoughts on why I choose to avoid it at all costs. Despite the fact that you can run some Windows software without needing to bother with a Windows OS installation, entirely too many software titles don't work as expected. Or worse yet, they work in one instance and later break after a WINE update. I've found that WINE works okay for some Windows video game titles and select Windows Office suite releases. Outside of this, I would suggest looking to a virtual machine instead, as you will find it's much more reliable overall.
When it comes to using virtual machines on Ubuntu, there are no absolutes. You simply need to consider the points made above, what functionality you're looking for and why you want to use a virtual machine in the first place. I would also like to point out that many of you will flat out disagree with some of my recommendations or perhaps feel I've been biased somehow.
For those of you who feel I might be mistaken, I will remind you of the following: everything above is based on my experiences using the listed virtual machine applications running Ubuntu as the host OS with Windows 7 as the guest. When evaluating both cost and ease of use, there can be no question that for the casual user VirtualBox is a clear winner, while VMware is the absolute best choice for the Ubuntu enthusiast looking for extra functionality.