You can hit Menu>Switch Application. It shows you the applications that are currently running, i believe there are 5 native/default ones.. for the other ones you should switch to it and press the menu button and click close to fully close the applications. Alternatively while Holding down the Alt key tap the escape key. This is basically the same thing as switch applications screen. Any program other than Messages, Browser, BB Messenger, Phone and home screen that is running can be shut down to gain some additional resources.
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