If you use Gnome, Shotwell will be one of your favorite photo viewer. This simple, minimalistic application that has the ability to do all you want is soon going to get a face detection tool, thanks to a developer working on it as his Google Summer of Code Project.
The developer writes, "The next stage of this project, and probably the harder one —and I need to improve this face detection suite to actually see the results, because it would be really sad to integrate this in Shotwell and later find that it is useless."
Using a program called facedetect-test, users can perform face detection using multiple cascade files and multiple values and the detected faces will be marked with red rectangles.
The project is still at nascent stages and a test suite is being developed. One can expect, for the time being, Shotwell will have a face detection software in the near future.