U Better Sing! is a really neat sight-singing app trainer. It is challenging enough for any singer to get something out of it, but accessible enough for younger students to enjoy. I don't typically teach voice, but one of my students is really motivated by singing, so we have been doing some voice lessons for the past 6 months.
This app is great. It uses a fun cartoon environment to teach the basics of solfege singing. The students earn and unlock new levels. There is a microphone input so the app can tell if you are matching pitch. There are also listening clips to make sure the student can hear what the notes will sounds like.