My personal portfolio as a pianist
Piano studio youtube channel
This is a very useful library of sheet music you can download and print. (Has music that is no longer under copyright)
Dictionary for looking up musical terms
A site that teaches theory (more geared towards adults than kids)
Susan Paridis free piano teaching resources including music, games, worksheets, teaching ideas
Compose Create's Downloadable Piano Teaching Resources
Lots of music theory exercises and games
I especially like this site's composer game time period game.
Free printable resources for teachers/students
Online Music Theory Flashcards
If you search music theory flashcards or music note flashcards, there are a lot of available quizlets for you to test your knowledge and some fun games to go with it!