What machines will OS X 10.10 (Yosemite) run on?
Business Insider lists the following Macs as compatible : 


iMac (mid-2007 or newer)
MacBook Air (late 2008 or newer)
MacBook (Late 2008 Aluminum, or Early 2009 or newer)
Mac Mini (Early 2009 or newer)
MacBook Pro (Mid/Late 2007 or newer)
Mac Pro (Early 2008 or newer)
Xserve (Early 2009)