Tools for Tantra
This book by Harish Johari describes Tantric Yantras, the geometric forms used in Hindu worship.
http://www.insight-books.com/meta/0892810556mt.html

Shiva Shakti Mandalam
Information on the Natha, Kalikula - Kali, Srikula - Lalita (Srividya) traditions, Tantrik rituals, and texts and translations of Tantrik literature.
http://www.shivashakti.com

Thirumandhiram
Tantric text in Tamil written by Tirumoolar. Includes ashtanga yoga, many yantras and philosophic discussions.
http://www.shaivism.org/thm.html

Tantra: Basics
Tantrik Aghorinathji explains the basics of Tantrism, the Tantrik approach to life, its uses and abuses and Tantrik Yoga.
http://hinduism.about.com/library/weekly/aa082501a.htm

Tantrism and Neo Tantrism
An article by Georg Feuerstein, Ph.D. about traditional Tantrism and its modern derivative Neo Tantrism.
http://santosha.com/moksha/tantrism1.html

Tantric teacher Eric Baret
Provides information on the seminars and books from this contributor to Shaivite Tantrism.
http://www.bhairava.ws/english/english.html

Tantra: Colorado University
Research paper on Andre Padoux's writings on Tantras from the Department of Religious studies.
http://www.colorado.edu/ReligiousStudies/TheStrip/features/thesis/academics/padoux/pad1.htm

Wikipedia: Tantra
The history, practice, identification with Deities, and the Hindu tradition.
http://en.wikipedia.org/wiki/Tantra

Hindu worship: Mantra, Tantra and Yantra
An article explaining the role of mantra, tantra and yantra in the worship of the Hindus.
http://www.hinduwebsite.com/secretsofworship.asp

Tantra Mantra Yantra
A short article on Tantras.
http://www.taantrik.com