For lollipop and above you can use elevation.
For older versions:
Here is a lazy hack from: http://odedhb.blogspot.com/2013/05/android-layout-shadow-without-9-patch.html
(toast_frame does not work on KitKat, shadow was removed from toasts)
just use:
android:background="@android:drawable/toast_frame"
or:
android:background="@android:drawable/dialog_frame"
as a background
examples:
<TextView
android:layout_width="fill_parent"
android:text="I am a simple textview with a shadow"
android:layout_height="wrap_content"
android:textSize="18sp"
android:padding="16dp"
android:textColor="#fff"
android:background="@android:drawable/toast_frame"
/>
and with different bg color:
<LinearLayout
android:layout_height="64dp"
android:layout_width="fill_parent"
android:gravity="center"
android:background="@android:drawable/toast_frame"
android:padding="4dp"
>
<Button
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:text="Button shadow"
android:background="#33b5e5"
android:textSize="24sp"
android:textStyle="bold"
android:textColor="#fff"
android:layout_gravity="center|bottom"
/>
</LinearLayout>