i have been using vodafone it offers 150MB data for 18rs with 3 days validity and 2GB data for 98rs for 30 days validity which is not a bit cost, the data is sufficient (Since no great speed compared to broadband).
Aircel has unlimited where it offers 14rs for 3 days & 98rs for 30 days
Tata Docomo had some ne offers check out their sittes for more info