Top of the list has to be Spotify, I don't like listening to the same music over and over, and Spotify's all you you eat music streaming service is almost perfect. I only use the default music app for my collection of rare bootlegs, I hate having to sync music to devices, with Spotify I can just find playlists shared by friends or strangers and listen to them instantly.
I couldn't live without the chess app, the ability to find an online opponent 24/7 is an option I exercise regularly.
Recently I a rowdy chess player was sending me abusive messages, which on most online communities would put me off, but after pushing him into an inevitable checkmate, I was able to wind him up with a few taunts of my own, that made the win even sweeter.
Last but not least is Strava, an app that times and tracks all my cycle rides. It encourages me to push myself as each part of my ride has a leaderboard with a hundred or more other cyclists in it, and I've slowly been making my way up the table, and burning calories along the way.
