The next version of OS X is going to be released today (30th September 2015) no word on actual release times from Apple.
OS X El Capitan builds on the groundbreaking features and beautiful design introduced in OS X Yosemite, refining the experience and improving performance in lots of little ways that make a big difference. OS X El Capitan — it takes the Mac experience to new heights.
We are really excited for this to be released, El Capitan will be a free upgrade as was Yosemite! El Capitan comes with a whole host of new features and upgrades.
The complete list of compatible machines looks like this:
MacBook (13-inch Aluminum, Late 2008), (13-inch, Early 2009 or later), (12-inch, Early 2015)
MacBook Pro (13-inch, Mid-2009 or later), (15-inch, Mid/Late 2007 or later), (17-inch, Late 2007 or later)
MacBook Air (Late 2008 or later)
Mac Mini (Early 2009 or later)
Mac Pro (Early 2008 or later)
iMac (Mid-2007 or later)
For all the preview details check out Apple's preview https://www.apple.com/osx/elcapitan-preview