Siddhar

The Siddhar (Tamil: சித்தர் "cittar", from Sanskrit: "siddha") in Tamil tradition is a perfected individual, who has attained spiritual powers called "siddhi".
Historically, Siddhar also refers to the people who were early age wandering adepts that dominated ancient Tamil teaching and philosophy. They were knowledgeable in science, technology, astronomy, literature, fine arts, music, drama, dance, and provided solutions to common people in their illness and advice for their future. Some of their ideologies are considered to have originated during the First Sangam period.
Practice.
Typically Siddhars were first Scientists, saints, doctors, alchemists, and mystics all in one. They wrote their findings in the form of Tamil poems on palm leaf manuscripts. These are still owned by some families in Tamil Nadu and handed down through the generations, as well as being kept in universities in India, Germany, Great Britain, and the United States.
In this way, Siddhars developed the native Siddha medicine system. A rustic form of healing that is similar to Siddha medicine has since been practiced by experienced elders in the villages of Tamil Nadu. This is referred to as "Paatti Vaitthiyam" (grandmother's medicine) "Naattu marunthu" (folk medicine) and "Mooligai marutthuvam" (herbal medicine).
Siddhars are also believed to be the founders of Varma kalai - a martial art for self-defense and medical treatment at the same time. Varmam are specific points located in the human body which when pressed in different ways can give various results, such as disabling an attacker in self-defense, or balancing a physical condition as an easy first-aid medical treatment.
Tamil Siddhars were the first to develop pulse-reading ("naadi paarththal" in Tamil) to identify the origin of diseases.
Siddhars have also written many religious poems. It is believed that most of them have lived for ages, in a mystic mountain called Sathuragiri, near Thanipparai village in Tamil Nadu.
Siddhars.
The Abithana Chintamani encyclopedia states that the Siddhars are either of the 9 or 18 persons listed below, but sage Agastya states that there are many who precede and follow these. 
The 9 Siddhars.
The 9 listed in "Abithana Chintamani" are as follows: 
The 18 Siddhars.
There are 18 siddhars in the Tamil Siddha tradition. They are
Apart from the 18 siddars listed above, there is another list of 18 siddars who represent the 9 navagrahas (two siddars represent each navagraha) all navagraha doshas /pariharams are performed to the siddars as Siddar Velvi (siddar havan) .The details of the 18 siddars who represent the 9 navagrahas are given below 
1.Sri Siva Vakya Siddar - Moon
2.Sri Kailaya Kambili Sattai Muni Siddar - Moon
3.Sri Bhogar siddar - Mars
4.Sri Kagabhujanga siddar - Jupiter
5.Sri Pullipanisiddar - Mars
6.Sri Sattai Muni siddar- Kethu
7.Sri Agapaisiddar - Jupiter
8.sri Azhugani siddar -Raghu
9.Sri Kudambai siddar - Kethu
10.Sri Vallalarsiddar - Mercury
11.Sri Edaikaddar siddar -Mercury
12.Sri Pattinathar siddar- Sun
13.Sri Kaduvelli siddar- Sun
14.Sri Kanjamalai siddar - Venus
15.Sri Sennimalai siddar- Venus
16. Sri Kapilar siddar -Saturn
17.Sri Karuvoorar siddar-Saturn
18.Sri Pambatti siddar -Raghu
There is a universal shrine for all the 18 Siddars at Madambakkam in Chennai called SriChakra Mahameru Sri Seshadri Swamigal 18 siddars Vrindavana Sakthi Peedam built under divine instruction from Sathguru Sri Seshadri Swamigal by Guruji. SHARMA JI.
The Buddhist monk called Bodhidharma is also a Siddar because has the power of Varman 
The supreme Siddhar is Lord Shiva himself.
Powers of siddhas.
The siddhars are believed to have had both major and minor powers which are described in detail in various yogic and religious texts. They also are said to have the power of converting their mass to energy and thereby traveling to different universes.
These eight are the Great Siddhis (Ashtama siddhis), or Great Perfections.