The My Jewish Learning site provides basic information about Judaism and the Jews, divided into the following categories: History & Community, Daily Life & Practice, Holidays, Life Cycle, Texts, Ideas & Beliefs, Culture. Developed by Hebrew College and Jewish Family & Life, this site is an ideal starting place for learning about things Jewish.
Judaism 101 is another excellent starting point for information about Judaism - people, places, ideas, beliefs, holidays, calendar, life events, and much more.
This Hebrew site - Ma’agar Sifrut Ha’Tanach (Holy Text Resources) - includes texts from Torah, Mishnah, Talmud and Rambam’s Mishneh Torah. Most importantly, it has links of references from one source to another. Start from the Torah and find where a pasuk is referenced in the Talmud, or start from the Talmud and link back to the original source. A very excellent resource.
The ORT site, Navigating the Bible: Online Bar/Bat Mitzvah Tutor, includes all weekly Torah readings - in Torah script with vowels and without, in translation, in transliteration, with audio files of each pasuk. Haftarot with translation, brachot before and after the Torah readings and Divrei Torah are included. The tools section includes such items as a calendar to determine the weekly reading based on your birthdate, a reference guide to people, places, plants and animals in the Torah, Ta’amei Hamikra audio files and a Biblical family tree.
A Page of Talmud is a wonderful resource both as an introduction to the Talmud, and for those who already have some background. The site includes a "Talmud Page Image Map": click on any section and get a description of that section, including biographical material on commentators, background information and much more.
The Jewish Parent Page by The Union of American Hebrew Congregations includes information about the holidays as well as projects, games, songs and more.
The Union of Orthodox Jewish Congregations of America has put together a wealth of information on the holidays, including extracts from various sources and audio clips.