A first alpha build for 10.0.0 has been published.
You can download it here for testing or development purposes: https://download.owncloud.org/community/testing/owncloud-10.0.0alpha.tar.bz2
Here is a list of new features in this build: https://github.com/owncloud/core/wiki/ownCloud-10.0-Features.
The new feature custom groups is also available as a separate app: https://github.com/owncloud/customgroups/releases/download/v0.1.0/customgroups.tar.bz2
Please note that the marketplace is not ready yet so third party apps will currently need to be installed manually by extracting them in the "apps" folder. In the future it will be possible to install all apps from there from within ownCloud.
Please let us know about issues by posting reports in the bug trackers.