I posted in this thread a while back. My selection of apps hasn't changed much:
500 Firepaper - Automatically download and cycle wallpapers from various categories.
BallMaze - Nifty little puzzle game. Not too difficult but it makes you think.
BitTorrent Sync - Near-perfect solution for backing up & synchronizing files between PCs and devices. Early versions were ridiculous battery consumers, so I configured Tasker to launch the app and terminate it automatically.
Chess By Post - I own an Android device. My brother has a Windows Phone. My mom has an iPhone. This app lets us all play chess together.
Wolfram Alpha - Invaluable if you're taking any math or science course in college.