Android : extract apk

http://android.stackexchange.com/questions/49114/how-do-i-save-purchased-apps-as-apks
http://www.quora.com/Why-dont-Android-app-stores-allow-saving-of-apk-files-to-our-local-disk

If you have a look at /data/app folder you can see apk files of installed apps.
In case your device is not rooted, you cannot see content of folder but you can "adb pull" any apk file if you know full name of the file.