iOS: If you want to really kill it at karaoke, approximately hitting the notes won't be good enough. A free app called Vanido can guide you through singing exercises, and show you in real time how well you're hitting those notes.
First, though, you get to find out what your range is, and how it compares to terms like bass, tenor, soprano and so on. (I'm a mezzo-soprano. I feel fancy already.) Once the app knows what notes you can sing, it can give you your daily voice exercises.
You only get three per day, but if you come back tomorrow there will be a different selection of exercises. Or, as the app calls them, games. Each one is a little riff that you sing: "Ya ya ya ya ya," for example, the tones going up and then down again. And then the app gives you a slightly higher note to start on, and you keep going for maybe a dozen repetitions.
If you've ever taken voice lessons or practised with a choir, some of these exercises will sound familiar. Others are more fun, like the "Bumblebee" one that zigzags quickly up and down. There's plenty of variety, so you won't get bored.
You need to use the app with headphones: It plays piano notes through the headphones, and then uses the microphone to listen to your voice. (You'll want to do your singing lessons in a quiet place.)
The best part is the real-time visual of what your voice is doing. If you go a little too high or too low as you approach the note, you'll see it on the screen. That's both the strength and the weakness of this app: You can get lost in watching the little squiggly line, and end up paying more attention to it than to how you sound.
You're totally on your own to decide if you like how your voice sounds. The app doesn't play you any examples of good singing, and it can't advise you on anything other than the pitch you're hitting. Vanido's creator wrote to say: "We are planning to add real voice samples, more guidance on technique, and breathing warmups in the coming updates."
But if you need to practise singing accurately, with immediate honest feedback, Vanido is a great pocket voice coach.
Vanido (free) [iTunes]