Some time in the last 10-15 years, the word has come to mean a sort of snug sock cap, popularized (I think) by snowboarders and the like, and then adopted by people who want to look like snowboarders:
Now it's become a default part of the wardrobe of men under the age of 50.
(the watermarks were included free with this clip art!)
I have friends who wear them, so I know I'm probably treading on some toes here, but I just don't like 'em! I don't like their ubiquity, over-casualness, or the way they often make the wearer's head look too small. The default position seems to be to pull the beanie all the way down to the wearer's eyebrows, which reduces facial expressions down to one look of blank surliness:
(I know, I cheated by using a photo of Eminem to demonstrate that point)