Guest SMP support has been a feature that's long been lacking from VirtualBox to provide better multi-core support, but it's finally arrived! To use the guest SMP support in VirtualBox, a processor with VT-x or AMD-V is required. It's also exciting to see OpenGL 2.0 now being supported across Linux/Windows/Solaris guests. It was just late last year when VirtualBox received OpenGL acceleration and then in January when it supported Direct3D acceleration. This hardware graphics acceleration was initially limited to Windows guests, but in April they finally pushed out Linux 3D support and now in June we have OpenGL 2.0 support. Of course, the host OS must also have OpenGL 2 capable hardware and drivers installed.
Beyond these major changes that have worked their way into the first VirtualBox 3.0 release, there is also various bug fixes that have went in since the VirtualBox 2.x branch too. The VirtualBox 3.0 Beta 1 release announcement can be read on the project's mailing list.