Oracle VirtualBox 4.0 brings many improvements to this virtualization stack they received as part of their Sun Microsystems acquisition. There's now the concept of extension packs, 2D/3D acceleration improvements, many bug-fixes, Java bindings, support for new virtualized hardware by the guests, and lots more.
The press release announcing VirtualBox 4.0 can be found in the Oracle news room. There's also the mailing list announcement with download links for Linux, Solaris, Windows, and Mac OS X operating systems.
Earlier this month we delivered our initial VirtualBox 4.0 benchmarks including a look at the virtualized OpenGL performance.