http://www.hinduwebsite.com/hinduism/essays/siddhas.asp
 
has an essay, "A critical study of the chronology of the Siddhas" by K V Ramakrishna Rao. This is basically from a Tamil Siddha tradition point of view.
 
http://www.siddhars.com/aboutus.htm
 
has a list of 18 Siddhas.

--
Prof.Nagaraj Paturi
Hyderabad-500044