Parallels Before VirtualBox became a popular option, there was Parallels Workstation for the desktop Linux user. I have found that upgrades to new versions of this VM solution has seen less regression issues than with VirtualBox. But others will likely see this differently. I am merely speaking of my personal experience.
Features provided by Parallels Workstation generally mirror those of VirtualBox. Outside of the occasional report that it allegedly runs better with AMD CPUs than the VirtualBox or VMWare, I have found it to be generally lackluster based on my ongoing trials with it.
Key features include (Excluding features already mentioned with competing VirtualBox)
Supports up to five network adapters at the same time.
Support for Ubuntu 7.10 and Fedora 8. No, this is not a typo...according to Parallels, supporting outdated Linux releases is apparently a "feature."
Change between Wi-fi to wired connections on the fly.
NetBSD and OpenBSD are now able to be installed as a guest OS.
Support for Windows 3.1 up to Vista supported for guest OS'.
While I cannot speak for everyone, I am less than impressed with the emphasis on outdated operating systems. Windows 3.1 and Ubuntu 7.10 support? Clearly VirtualBox and VMWare should be concerned...
Which virtualization solution is best for your desktop?
There is certainly no blanket answer for best desktop virtualization solution. That said, I would acknowledge the following price, functionality and stability are king. This holds true for both the casual home user in addition to that of enterprise user. There are defining differences in who needs what, which brings us to the following conclusion.
For the home user and to some extent, most SoHo users, VirtualBox is a great fit. Cost effective, fast and generally stable to use with some initial tweaking, this VM option provides the kind of balance that most people in this user group will be looking for.
On the enterprise front however, there is no question whatsoever that VMware Workstation will remain king of this domain. From access to professional support for troubleshooting to enterprise level stability/predictability that allows the existing IT staff to better manage virtual OS installs with ease, VMWare wins the day for the enterprise, hands down.
Then we have Parallels Workstation, along with other virtualization solutions that I did not choose to mention today. I believe that Parallels will be a good fit for home and SoHo users that found VirtualBox was simply not meeting their needs. There is no question that this happens on occasion, so having a viable alternative does help make virtualization a feasible reality for those without a desire to jump into the VMware Workstation frontier.
What of the other solutions not mentioned? Qemu is one option for those who are very comfortable with getting into their Linux installation at a deeper level than required with solutions presented above. Also, an option like Win4Lin limits you to hosting Windows guest operating systems only.
Obviously, trying out what appeals to you based on price and features offered is the most logical course of action. In the end however, most users will find that VirtualBox is the logical choice for the masses at this point in time.