Most prominent in the frontman of the Foo Fighters
Dave Grohl (and thusly named after him), the Dave Grohl Effect is when a man looks far, far better when he has a beard. Examples of more people the Dave Grohl Effect works on are Sean Bean, Alec Baldwin, Will Wheaton, Martin Freeman and David Schwimmer.
"Hey, have you seen Joe? He's really rocking that new beard, makes him look totally badass"
"Yeah, that'll be the Dave Grohl Effect."