Windows/Mac/Linux: XBMC media center has dropped a 9.11 alpha release for all platforms, with quality fixes on all three computer platforms and more powerful skinning abilities. If you're not afraid of a bug or two, it's yours to grab.
From the outset, not a whole lot looks new, but XBMC has made some real improvements under the hood. From the developer's release notes:
DirectX has replaced OpenGL for Windows
Numerous skinning engine updates which allow for use of some fancy new skins
Many improvements to the video player which allow for much smoother playback
Switch to Core Audio on OSX
Many VDPAU improvements under linux
Many platform-specific customizations
There's a promise for "much much more" to be revealed as the final release approaches, but core video and audio improvements are always welcome among media fans.