Here are some potentially useful sites:

RaMBI (Maintained by the Jewish National and University Library [JNUL] at the Hebrew University in Jerusalem, it is the best research tool for periodical literature in Jewish Studies; also known as the Index of Articles in Jewish Studies)
The Jewish Encyclopedia Online (Published between 1901-1906 in 12 volumes, it was the scholarly standard for the first half of the 20th century. Most of the articles about Jewish mysticism are dated [all written before the work of Scholem], but still useful)
The Ten Sefirot of the Kabbalah (An interactive symbol decipherer; from E. Segal's site)
The Tree of Life, the Ten SefirotMy version
Journal des Etudes de la Cabale / Journal of Kabbalah Studies (mostly in French, some English, some Hebrew) This is the best scholarly site on the WEB
The Kabbalah Centre (This is the main web page for the Bergs' pop-Kabbalah empire)
Some excellent bibliographical guides by Don Karr
The Zohar, Pritzker edition, companion web site (includes samples, and the complete Aramaic text)
A section of Sefer ha-Bahir
Kabbalistic texts in Hebrew A Czech site with a number of important texts, including Hekhalot Rabbati, Sefer Yetzirah, Sefer ha-Qanah, Sefer ha-Peli'ah, works by Abraham Abulafia, Joseph Gikatillia, Moses Cordovero, and Hayyim Vital
A Letter from R. Isaac the Blind (Tr. by G. Scholem)
Other links

From a page of Meir Poppers' Ilan ha-Gadol

Two Idiots Discuss Kabbalah (a joke)