This website has practice exercises for everything from note dictation and clef reading to harmonic progressions and the construction and identification of seventh chords. The "exercise" page, which this link goes directly to, houses all of the practice resources.

"Cantorion is a free sheet music repository and a free concert listing diary that anyone can contribute to." This website is a good resource for written music and may assist in written theory and recognizing intervals. Also a great source for free piano music!