Tesoros de Lectura from McGraw-Hill Publishers. This Spanish web site contains pages with information and links to resources for teachers, parents, and students, but the prime focus is on a series of reading comprehension units for each grade. Each unit focuses on a different theme (i.e., la familia, los alimentos, el tiempo y las estaciones, etc.) and contains a collection of multimedia, minibooks, and activities, as well as discussion topics.
If any of you try these out, let me know what you think. I am going to download the first, but it may be too advanced for my children.