I have created one application on my N900 and wanted to take screen shot of that application while it is running on N900. I tried to search such application on application manager but was not able to find such application. So i tried to Google for this and after searching while I found that N900 already have screen capture utility installed and to capture screen there is special key combination. Key combination is CTRL + SHIFT + P. By pressing this key combination this utility will capture screen and save image in /Images/Screenshots folder.
Here is my sample screenshot.