PracticeSpot.com is a well-stocked site with resources for practicing, aimed at both music teachers and students (it is not specific to piano). The founder, Philip Johnston has published several books on practice, available for purchase here. But there are many free resources: drill sheets and exercises for chords, scales, pitch recognition, sight reading, etc. A collection of well-written and, from what I’ve seen so far, sensible practice tips and coaching (the Infopedia). Performance advice, practice strategies, and more.
Johnston has recently published a new book called Practiceopedia: a 376 page reference on practice techniques, written specifically for students. I got a copy of this (being a practice advice junkie), and I think it is quite good. The basic theme is that you can learn faster and better if you use good practice techniques: nothing unfamiliar there to serious students, but it is packed with concrete advice, strategies, and insights. For some reason I can’t find this book listed at PracticeSpot.com (though it’s published by PracticeSpot Press), but it has it’s own web site: http://www.practiceopedia.com.