Depends on your purpose -
For textbooks (to learn basic well established facts about a topic): Zitelli's Atlas (the best all around peds book available), Red Book (for ID topics), Long's pediatric infectious diseases, Park's Pediatric Cardiology for practitioners (a gem of a book), Habif's Clinical Dermatology.
For quick reference (running to an admission): Up to date, Emedicine
For board studying: Peds in review, Zitelli's Atlas (again)
For in-depth literature review: Start with MD Consult and follow the references - Journals you will be guided to include Pediatrics, J. Peds, NEJM, The Lancet.
To learn a lot about a single topic (you're presenting for morning report, etc): Clinics of North America
