Nova Launcher: A fast, fancy, customizable launcher... (you can assign 'activities' or create shortcuts for them) [ if your stock rom not allowing you to move apps icon in home eg: samsung ]

Switchr: Alt-Tab like task switcher. Useful when screen is not big and/or you want fullscreen appls

Floating Apps: As name defines, have some floating windows like browser notepad etc... Is useful when taking notes...

3D image wallpaper: Creates a movable wallpaper behind desktop and menus. When you move your phone its moving slightly, creates nice effect...

Facebook Messenger: Popped out chat heads...

C4Droid or CppDroid: Write, edit and compile C programs on your droid!

Conncectbot: Is a telnet ssh client and terminal emulator (if you connect local)

Flipboard: Digital magazine / News... Im following Tech/Science section... Its very nice and readable...

Remote keyboard: Connect to your phone through telnet and what you write is what you see... (Use pc as keyboard to android)

Googles: OCR + QR-code scanner... Also does google search for it... Identifies pictures you have taken. For example Im in Turkey/Istanbul... When I take picture of beach from middle of bosphorus It identifies area (example Besiktas Ortakoy) place names etc.

Camera FV-5: Turn your smartphone into DSLR-Like thing... Its not good as real DSLR but you can take long exposure shots, change focus point etc... Boosts performance at night shots

Remote droid: use your phone/tablet as touchpad for your computer... Requires java-based client on computer...

Gravity Screen: A lot of nice features/gestures... Phone auto on/off when put on desk and taken to face up. A setting to not to enter sleep when its vibrating (when you holding in your hand, its moving so little but that app detects it and not allowing screen to turn off when you reading something... Better than samsung's shitty smartstay)

X-Plane: Aircraft simulation game... I like to mess around :)