PFG iBroker is a great Android mobile trading application.
The application gives good quotes on an easy-to-read interface and allows one to trade several different types of derivatives: futures, options, & forex.
This application also allows users to create paper-trading accounts for free. Paper trading is the simulated trading of securities or derivatives with pretend money. This is usually done to test one's skill, trading strategies, automated trading strategies, etc.
Below is a screenshot of a fill order I received from shorting 15 S&P 500 futures contracts for June 2011 expiration in my paper trading account.