VirtualBox is an open source application to manage and run virtual machines. If you ever need to run a virtual (guest) operating system on your host machine that runs Windows, Linux, Mac OS X or Solaris operating system, you should check this program out. It is absolute free but includes almost all the features that the modern virtual machines must have.
Innotek, the original developer of the software was bought out by Sun Microsystems. In early 2010, one of the top 100 Fortune companies, Oracle, bought out Sun and is now the current development curator of this application.
Given the open source nature of the software, it is expected that it is available for multiple platforms. One can download the program for absolutely free from the program's website. One can also find an extension pack available for a free download that enables the support of USB 2.0 and other technologies under guest operating system.
There are some bugs here and there, but if you check out the development logs, you can see that the developers address those bugs pretty quickly in hopes that they won't loose the already existing valuable customer base. There were only 5 months in between two major releases, so you can see that the ball is rolling and the application is not abandoned but evolving.
As far as the features go, the virtual machine market is very saturated with open source application, hence all of the VM developers are trying to keep up with the latest trends, giving the customers support for the latest technology. VirtualBox supports VT-x and AMD-v, therefore increasing the execution speed of your guest operating system. No word on supporting of VT-d, but I am sure it is coming in the near future, as the chips with that extension catch up the market. Keep in mind that using VirtualBox gives you an edge as it supports other virtual machine's hard drive images, such as VMWare's and Microsoft's, while the relationship is reciprocal due to close source nature of VMWare and Microsoft's products.
Overall, this application is one of the leaders on the market for virtualization. Its open- source nature gives it an edge over the closed source applications in regards to implementing the newest code and technologies.