Android-x86 4.4-r4 was just released a few days ago as the latest stable release while Android-x86 5.1 is still in a release candidate state. In early December was a test release of Android-x86 6.0 while out today is a new 6.0 release to succeed it.
android-x86-6.0-20160127 is the new test release available today. This new test release can be downloaded from SourceForge but so far I haven't been able to find any official change-log / release notes.
Android 6.0 "Marshmallow" was officially released last October with UX improvements over Android 5, new APIs, a new permissions architecture, native support for fingerprint recognition, and a variety of other enhancements.