Which Macs are compatible with macOS Sierra?
The Keynote announced the following supported hardware: 


Late 2009: MacBook (MacBook6,1)
Late 2009: iMac (21.5" iMac10,1, 27" iMac10,1 and iMac 10,2)
Mid 2010: MacBook Pro (13" MacBookPro7,1, 15" MacBookPro6,2, 17" MacBookPro6,1)
Mid 2010: Mac mini (Macmini4,1)
Mid 2010 : Mac Pro (MacPro5,1)
Late 2010: MacBook Air (11" MacBookAir3,1, 13" MacBookAir3,2)


All later models are unquestionably compatible. A public list (on Apple's website) is not available yet, but a list may be available in the Release Notes (Developer Only). 

For the Developer Preview: Registered Developers can access InstallableMachines.plist in InstallESD.dmg for a full list of officially supported Mac board-ids.

It may be possible to install on some unsupported hardware through modifying firmware, the installer, or other methods.