RabLab Online--This is the website of the Rabbinics Lab of the Solomon Schechter Day School of Greater Boston. Hebrew texts and translations with student commentaries are available on the topics of Shabbat, Tzitzit, Tefillin, Mezuzah, Business Ethics, Medical Ethics, Ethics of Language, Tzedakah and Midrash. Hebrew is in graphic mode, so all browsers can use this site.
Tanakh, Mishnah, Tosefta, Talmud Bavli, Talmud Yerushalmi, Mishneh Torah of the Ramba"m--These texts are available online from the Hebrew University in Hebrew. This site requires Hebrew fonts installed on your browser (Internet Explorer 5 and Netscape Navigator 4 have Hebrew display capability built-in.)
Tanakh and Mishneh Torah online with translation of Mishneh Torah on the way, available from Machon Mamre. They also have a much broader selection of texts available for download to be accessed from DOS. The text of the Mishneh Torah is based on Yemenite manuscripts and is reportedly the most accurate text available (electronically or in print).
Kitzur Shulhan Arukh--A translation of a significant portion of the Kitzur Shulhan Arukh can be found at this URL.
Ramba"m's Sefer ha-Madda--A translation of the first of the fourteen sections of Maimonides' Mishneh Torah. Sefer ha-Madda includes material about Maimonides' theory of human behavior as well as rules about idolatry and repentance.
Reform Responsa-- The Central Conference of American Rabbis has posted an outstanding collection of contemporary English Responsa online.
Gates to Jewish Heritage--Rabbi David Lipman's site includes lots of selections of Rabbinic texts in translation archived thematically with a variety of different ways to enter into the texts.
Torah Reading and Haftarot--The complete text of the Torah and all of the Haftarot with vowels and also without (as it appears in the Torah) and transliterated and translated with RealAudio so that you can hear the text chanted with trope. (Okay, it's not Rabbinics, but it is really cool!)
Gemara Berura is a site which supports the use of computer based graphical tools (colors, indentation, flow-charts) to facilitate student-directed learning of Talmud. The site is not complete, but you can get a sense of the methodology from the materials that are currently available.
General Tools
WebShas--This is a topical index of the Talmud Bavli produced by Rabbi Mordecai Torczyner. Follow its menus until you find the proper reference and then pull the book off the shelf or type the reference into the Judaic Classics Library or Soncino Classics Collection.
HaMakor--This tool provides references from all of Rabbinic literature on selected topics. By the author of WebShas.
Rambam-- This tool provides a general overview of Jewish Law as codified by Maimonides. A unique double footnote system includes references to the Mishneh Torah as well as to the Biblical sources of the law.
Navigating the Jewish Classics--Professor Eliezer Segal's imagemaps explain all of the different elements on a page of Talmud (and also of the traditional editions of the Mishnah, Mikraot Gedolot, Mishneh Torah, Tur, Shulhan Arukh).
Tools for Specific Topics
Tzedakah--This site includes texts from the whole range of Rabbinic literature in Hebrew (graphic) and English translation with some study questions. The material was produced by Jeffrey Spitzer.
Shemirat HaLashon-- This site includes a loose translation of almost all of the Hafetz Hayyim's classic works on the ethics of language.
Tekhelet (the blue dye for Tzitzit)--This is the site produced by the P'til Tekhelet organization which explains how the Tekhelet dye was rediscovered.
Business Ethics--This collection of actual contemporary responsa and references to the sources gives a good idea of how ancient texts are practically applied to modern situtations.
Business Ethics from Aish
Torah Ethics Project--This site addresses a variety of crucial ethical issues (civil responsibility, family conflict, disabilities, sensitivity to differences, etc.) and provides scenarios for discussions as well as a wide range of Jewish sources in translation.
Medical Ethics--This site was produced by Jeffrey Spitzer and includes links to a wide range of responsa and articles on Jewish medical ethics found all over the WWW.