My research so far:
OptionsXPress - with commissions of about USD 1.25/contract. USD 1K minimum account opening.
Interactive Brokers (IB) - with commissions of about USD 0.70-1.00/contract. USD 10K minimum account opening.
TradeStation - with commissions of about USD 1/contract. One point to note is that TradeStation's EasyLanguage platform is NOT a true API and orders can only be executed via EasyLanguage and it's not a full API as per the sales rep. So, if you want to execute more complicated trading strategies (e.g. if front month call IV exceeds) you won't be able to execute it there. USD 1K minimum account opening.
Lightspeed - with commissions of about USD 0.60/contract. USD 30K Minimum account opening for using APIs.