1. Music theory web sites
Musictheory.net: lessons, exercises ad tools (a very complete and easy to use site)
The Music Theory Minute: Free one-minute online help tutorials for music theory students
2. Diction, IPA, Sight reading
Antimoon: the sounds of English and the IPA (with audio examples)
Diktus: program for practicing one- or two-part rhythmic dictation and sight reading
GoodEarPro: is a combination of goodEar Intervals, Chords, Melodies and Chords — which are all apps that can also be purchased separately for $0.99 each. The interface is easy-to-use, and the settings allow you to customize how the app quizzes you on certain topics.
3. Vocal technique
Vocal pedagogy (wiki): excellent introduction, especially to issues of anatomy, and the nature of vocal sounds
4. Music Theory Apps
Theory lessons by music theory.net (iPhone, iPad $2.99): pocket-sized animated music lessons.
Tenuto by music theory.net (iPhone, iPad $4.99): Tenuto is a collection of fifteen customizable exercises designed to enhance your musicality. From recognizing chords on a keyboard to identifying intervals by ear, it has an exercise for you. Tenuto also includes five musical calculators for accidentals, intervals, chords, analysis symbols, and twelve-tone matrices. The app contains a variety of modules for practicing identification of notes, key signatures, intervals, chords, piano keys — both by sight as well as by ear. The graphics are attractive and the app is easy-to-use. The modules can be adjusted to adjust the difficulty of the activity. Tenuto is an excellent tool for beginner students through advanced students.
5. Sheet Music Reader Apps
forScore: (iPad, $19.99) This is a full-featured sheet music annotation, organization, rehearsal, and performance app. A must have for anyone using an iPad in BCE.
MobileSheetsPro Music Reader: (Android 2.3 +, $14.99) This is a solid sheet music reader with some annotating functionality. Better experienced on 7" or larger android tablets and on Kitkat or better.
6. Miscellaneous
Alexander technique: "the complete guide to the Alexander technique"