It's hard to say which free music-streaming service is really worth the hard-drive space these days, but Slacker stands out from the pack. For starters, its music library is nearly four times bigger than that of any other music-streaming app with unlimited free access. Slacker is also loaded with more than 100 pre-programmed genre stations, which is perfect for, well, slackers. And the app lets you customize stations if you want full control over what you hear — you can fine-tune your channels to play more of any kind of music you wish.