Bible Gateway

Internet Sacred Text Archive

Children of Abraham : Judaism, Christianity, Islam

Brill Islamic Studies Online

(Index Islamicus, plus fulltext of Encyclopedia of Islam, Encyclopedia of the Qur'an, and Encyclopedia of Women & Islamic Cultures)

Christianity

Cassell dictionary of Christianity

Christian Classics Ethereal Library

Dictionary of Third World theologies

The Oxford dictionary of the Christian Church

The Westminster dictionary of church history

Islam

The Oxford dictionary of Islam

Concise encyclopedia of Islam

A history of Islamic societies

The Oxford history of Islam

Muslim perceptions of other religions : a historical survey

USC-MSA Compendium of Muslim Texts

Judaism

The Oxford dictionary of the Jewish religion
Also available as an ebook.

Historical dictionary of Judaism

The Routledge dictionary of Judaism

Judaism : an introduction