Well, it was an opensource project acquired by Sun, that was acquired by Oracle. But I'm not into the selling and buying business, so I'll just get to the subject. VirtualBox has something that VMWare Workstation (let's say 7) doesn't. And it's called 3D acceleration. Well, I know that VMWare Workstation has 3D acceleration since the 6.x versions, BUT, some of the people tried installing Ubuntu as a guest on a Windows with VMWare Workstation host. After installing the guest, they tried enabling desktop composition - Compiz Fusion. And they had a surprise, telling them that desktop effects could not be enabled. Why? Because VMWare Workstation only supports 3D acceleration for a Windows-based guest. Which means that Linux-based guests are not supported. So this is where VirtualBox comes into place. Well, VirtualBox supports 3D acceleration for whatever guest operating system you'll have. In fact, VirtualBox allows you to set up virtual video memory available for the guest operating system, which VMWare Workstation doesn't.
So, the best solution for Linux-based guests if you need 3D acceleration, VirtualBox is the solution. It's a little buggy though, ...for example, when you change desktop theme in Ubuntu, the screen flickers, but it's not flickering after it's done changing the theme, and what the hell, you can enable compiz. :)