AMD has just unleashed its new Catalyst 14.1 Beta drivers, which include the first proper look at Mantle in the public space. There is a download that you'll need for Battlefield 4 to enable Mantle, too.
Another Mantle-capable game is Star Swarm, which is available on Steam, and includes some bigger performance improvements when using a CPU-limited system. AMD is claiming a gigantic 319% performance improvement when running Star Swarm on its Extreme setting at 1080p, using their A10-7700K processor.
This is the very first release of Mantle, so we should only expect things to get better from here on out. You can grab the new drivers here.