Search for "thinkorswim" in Android Market. Or click straight here: https://market.android.com/details?id=com.devexperts.tdmobile.platform.android.thinkorswim TOS has very powerful mobile platform including Android.
We do offer an Android Platform. You can see all of our mobile platforms here: http://individuals.interactivebrokers.com/en/pagemap/pagemap_mobiletrading.php
Android TWS is compatible with any android smartphone version 1.6 and higher. It should run smoothly on the Galaxy S 2.
I just posted on another mobile questions thread it might help you too. Lind-Waldock allows you to place a trade through their web site's mobile version: http://www.lind-waldock.com/platforms/lindwireless.shtml Cannon Trading offers iRunner which looks really cool from the screen shots they have: http://www.cannontrading.com/futures-trading/mobile/platforms.php NinjaTrader has a mobile app. It says something about Androids on the screen shots: http://www.ninjaterminal.com/ Hope that helps you out buddy.
I run the mobileTWS on my Samsung Droid Charge and it is pretty smooth. Once you set it up the mobileTWS is very fast -- at least on my Droid Charge. The only thing I wish IB would do is make the charts with candlesticks in stead of just lines. They even have their Booktrader with a mini-DOM. It is pretty good overall. I think IB should make a more full fledged app for Android Tablets since some of the tablets are so powerful like the Motorola Xoom.