Pandora, Poynt, Google Maps and Earth, and USA Today are useful to me. Of course there are also games like Angry Birds and Temple Run for time wasters. Zedge is a good one for wallpapers, ringtones, and notifications. I like it especially because when you download something, you can share it via text, email, etc. These are just a few I like.
I would say Pandora, but the new interface update looks like something out of Facebook and I'm not fond of it. Otherwise it's cool. One I like a lot is Screen Filter. It's like virtual window tint for your phone if using in low light settings and even the dimmest backlight setting is too bright.