Must Have apps in order:
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1) Advanced App Killer - This app nearly
doubles your battery life by killing all background apps. A Lifesaver!
2) Dropbox - 'nuff said
3) Tiny Flashlight - Turns on all camera LED lights to max brightness - works really well. The other flashlight apps unnecessarily turn screen brightness up as well... that absorbs double the battery life and really doesn't help much.
4) SourceLair -
C++ compiler. Works really well. I keep my programs in dropbox and fool around with them on the train sometimes with SL.
Non must-have apps. But I have them anyways

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1) RealCalc - IMHO the best calculator for android
2) ColorNote - a useful notepad
3) Kaplan GRE Vocab - yes I'm done with the GREs, but after receiving my verbal score this app is a no brainer for me...
Games I have
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1) TapFish - cute little fishtank game. They try to swindle you into spending real money. I purchase the fish that appreciate in value the fastest and resell them. My fish are worth over $1,000,000 as of yesterday

. You start with around $1,000. Not very productive but I only spend around 2 minutes a day on it.
2) Backgammon from AI Factory - My favorite game.
3) Yahtz Me - Yahtzee... the original poker.