The esoteric yogic philosophy of the Tamil Siddhas is known as Shaiva Siddhanta, but this is ony part of a larger Shaiva Siddhanta tradition. Shaiva Siddhanta means literally the agreed or settled view (siddhanta) of Shaiva doctrine. The name indicates that this was the predomeinant religious philosophy of the time.
Shaiva Siddhanta would seem to represent the basis of metaphysical system to Kashmir Shaivism (Pratyabhijna); the idea of 36 tattvas, various classes of consciousnesses, and so on. It would seem that Kashmir Shaivism, which appeared in the 8th to 10th centuries, developed from a synthesis of Shaiva Siddhanta metaphysics with profound nonduality realization, hence its emphasis on panentheistic transcendence.
In the south however, Shaiva Siddhanta merged with Shaivite devotionalism (bhakti yoga) and thus represents a very different form. The great yogi Tirumular brought to this theistic system various insights regarding alchemy, siddha medicine, and transmutation and divinisation of the physical body. As a result, the Tantra to the Tamil Shaiva Siddhanta tradition is very different to teh Kashmir Shaivism on the north.
Because I'm not an expert, and because I'm too lazy/busy/add excuse to write a whole page, quoting Wikipedia ( 17 August 2009) is as good or as incompetent as any option. So here is the introduction, which seems to be well written (and is stringly footnoted I'm pleased to see):
Shaiva (or Saiva) Siddhanta is a Saivite Hindu school that encompasses tens of millions of adherents, predominantly in Tamil Nadu and Sri Lanka. Today it has thousands of active temples there and dozens of monastic/ascetic traditions: twenty-five Brahmin families, the Adisaivas, are qualified to perform its rituals.
The culmination of a long period of systematisation of its theology appears to have taken place in Kashmir in the tenth century, the exegetical works of the Kashmirian authors Bhatta Narayanakantha and Bhatta Ramakantha being the most sophisticated expressions of this school of thought. Their works were quoted and emulated in the works of twelfth-century South Indian authors, such as Aghorasiva and Trilocanasiva. The theology they expound is based on a canon of Tantric scriptures called Siddhantatantras or Shaiva Agamas. This canon is traditionally held to contain twenty-eight scriptures, but the lists vary, and several doctrinally significant scriptures, such as the Mrgendra, are not listed. In the systematisation of the liturgy of the Shaiva Siddhanta, the Kashmirian thinkers appear to have exercised less influence: the treatise that had the greatest impact on Shaiva ritual, and indeed on ritual outside the Shaiva sectarian domain, for we find traces of it in such works as the Agnipurana, is a ritual manual composed in North India in the late eleventh century by a certain Somasambhu. After the twelfth century, North Indian evidence for the presence of the Shaiva Siddhanta grows rarer. The school appears to have died out in other parts of India even as it grew in importance in the Tamil-speaking south. There its original emphasis on ritual fused with an intense devotional (bhakti) tradition. The Tamil compendium of devotional songs known as Tirumurai, along with the Vedas, the Shaiva Agamas and "Meykanda" or "Siddhanta" Sastras, form the scriptural canon of Tamil Shaiva Siddhanta. Tirumurai is a twelve-volume anthology of the works of few among sixty-three poets, the Nayanars,,Manikkavacakar, Sekkizhar and Others.The Meykanda sastras are fourteen in number, authored by St. Meykandar and his disciples.
For more, follow the Wikipedia link above.
Alexis Sanderson, The Saiva Exegesis of Kashmir, pp.242-248 (in Tantric Studies in Memory of Hélène Brunner, edited by Dominic Goodall and André Padoux, Pondicherry, French Institute of Pondicherry and Ecole francaise d'Extreme-Orient, 2007.
Dominic Goodall, Problems of Name and Lineage: Relationships between South Indian Authors of the Shaiva Siddhanta, Journal of the Royal Asiatic Society, Series 3, 10.2 (2000).
Extant lists are presented by Dominic Goodall in Appendix III of Bhatta Ramakantha's Commentary on the Kiranatantra, Pondicherry, French Institute of Pondicherry and Ecole francaise d'Extreme-Orient, 1998, pp.402-417.
This is one of the few demonstrably pre-tenth-century scriptures of the Shaiva Siddhanta to have been completely translated into a European language: Michel Hulin, Mrgendragama. Sections de la doctrine et du yoga, Pondicherry, French Institute of Pondicherry, 1980, and Hélène Brunner-Lachaux, Mrgendragama. Section des rites et sections du comportement, Pondicherry, French Institute of Pondichery, 1985.
This manual, called the Kriyakandakramavali or Somasambhupaddhati, has been edited, translated and richly annotated by Hélène Brunner and published in 4 volumes from the French Institute of Pondicherry in 1963, 1968, 1977 and 1998.
S.Arulsamy, Saivism - A Perspective of Grace, Sterling Publishers Private Limited, New Delhi, 1987, pp.1
Gavin Flood, An Introduction to Hinduism, Cambridge UP, 1996, chapter seven. and A. K. Ramanujan (trans. and intro) Speaking of Siva, Penguin Books, 1973.