Supercell uses chunky, stacked font which is a cool T-shirt trend right now.
Sometimes, they have double meanings. HotelTonight's logo is both an "H" and a bed.
Clever icons look good and represent what an app stands for. Like Duolingo's owl, "Duo." Because owls are wise, and Duolingo teaches its users new languages.
Evernote's elephant logo is similar. Elephants, like Evernote's cloud notebook, never forget.
A ghost, like Snapchat's "Ghostface Chillah" icon, disappears just like Snapchat's photo messages.
Tinder, which is like Hot or Not, rightfully uses a flame.
Frontback, a photo app that captures two pictures at once, use a smiley face.
Quip's logo is both a Q and a pencil with paper to signify the document creation tool.
Thrillist's logo turns the two Ts into East and West on a compass.
Rdio's logo has a music note both as an O and in the O's negative space.
Soundcloud turns half of its cloud logo into audio waves.
Foursquare's logo represents both the game Foursquare and one of its flagship features, the check mark for check-ins.