Chromatic Tuner and Metronome
This app is recommended both as a tuner and a metronome. It includes both, and the amount of features that it packs make it well worth the $3.99 purchase price. Unfortunately, it is only available for iOS at this time.
Metronomes keep perfectly steady time. There are a number of inexpensive and free metronomes available for smartphones and tablets, or students can purchase standalone metronomes. They are invaluable to practicing correct rhythms and staying in the same tempo throughout a song.
This app is FANTASTIC! For $2.99 on the App Store, it includes a tuner, metronome, recorder (to record and listen back to yourself when practicing - a great self-assessment tool), and more. It even has a set-list feature where you can save tempos from different sections of your music and just hit that button to practice the section. Unfortunately, it is only for iOS at this time.
Pro Metronome is a great free metronome app, available for iOS and Android devices. This app doesn't have all of the same features as the apps above, but has many of its own that are noteworthy. Find it on the App Store and Google Play.
Metronome Online is a great resource if you are practicing near a computer or with a smartphone or tablet. It is a free online metronome that allows you to log in and track your practice,