Samsung Galaxy Ace - Screen Capture

In order to take a screen capture on your Ace

1. Click and hold the 'Back' button
2. Simultaneously press the 'Menu'
3. A popup announces that the screenshot has been taken and saved as an image file.
4. Navigate to the 'My Files' folder and locate a newly created folder called 'ScreenCapture' on the SD Card. You will find your screenshot in there.

This will work only on static screens. If you are in the middle of a game (Fruit Ninja!!) and press the 'Back' key, it will likely return to the main menu of the game and not provide an opportunity to press the Menu button. 

You can find apps (mostly paid) on the Market that will help you take screenshots of games. 
Look around and have fun!