KPN Travels is one of the most reliable luxury bus operators to regions in Tamil Nadu, Kerala and Bangalore.
I have never tried booking through these guys, but these guys seems to support a lot of transport companies to a wide variety of places in south and central india.
They cover most of Tamil Nadu and Bangalore.
This site is like redbus, which cater to a variety of bus operators, covering mostly South India
They include bus operators throughout India.
At this point of time, it caters to a few cities in Tamil Nadu only.
Covers South India
Covers major cities in Tamil Nadu and Karnataka
Connects places inside Tamil Nadu
Please also refer to other services listed in http://blogs.ibibo.com/premnewzea/Bus-Tickets-Online.html