TE Tuner: The Tonal Energy tuner is quite possibly my favorite app. As a musician it is the best four dollars you will ever spend. The opening interface is just a normal tuner. But it has settings so that you can adjust the frequency you're tuning at, and you change how sensitive it is (the more experienced you are, the more sensitivity you would want). But the best part of the app is the drone function. You can set it up to drone a note that you can play against, which is more valuable than just a tuner as it trains the players ears. You can set it up for concert or transposing pitch if your students don't quite understand what their pitch is supposed to be. It also has a function that allows you to see the overtones that are in the note you are playing.
Smart Music: Plenty of musicians and teachers already know about this program. It costs 40 dollars per year for students, or a 140 dollars per year for teachers. It has a metronome, a tuner, and a library of thousands of pieces, solo or ensemble. Students can use this program while they study their solos and play along with the recorded accompaniment. You can choose whatever tempo you want, set it up to follow along with the player or just play one solid tempo, and change tempos in certain sections. It also has sight reading exercises that you can have your students do so that they become better at it.
musictheory.net: musictheory.net is a wonderful resource for students. It has so many lessons on the basics of music like staff and note identification, and identifying various musical markings. The website is free to use, and there's an app you can purchase on your phone. It's great for beginning students, but it's also a great introduction into music theory for your more advanced students.