Here are a few web sites I have found useful.

This list is just a start.  Many offer free things, some offer a mix of free plus extras if you subscribe.

Sheet music:

http://www.8notes.com/piano/

http://www.gmajormusictheory.org/Freebies/freebies.html

http://makingmusicfun.net/htm/printit_piano_sheet_music_index.htm

Theory:

To practise aural work and theory topics

https://www.teoria.com/en/exercises/

To listen to music:

Classical:

http://www.classical.com

http://www.classicsforkids.com/

Contemporary

https://supersonicspiano.com

General Information:

Piano Technicians Guild - includes information on how pianos work, how to care for your piano

http://www.ptg.org

A wealth of information and resources for teachers, parents, students - everyone

http://pianoeducation.org/

Supersonics

Here is where you can listen to  the Supersonics Pieces you may be learning. Just click on the album the piece is from and mp3 tracks will be available:

http://supersonicspiano.com/