You can now use the FloatingActionButton from the support-design library. Add this dependency to your gradle build file:
compile 'com.android.support:design:22.2.0'
And then add the FloatingActionButton to your layout file:
<android.support.design.widget.FloatingActionButton
android:id="@+id/fab"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="end|bottom"
android:layout_margin="@dimen/fab_margin"
android:src="https://stackoverflow.com/questions/24605116/@drawable/ic_done" />
Example project from Chris Banes: https://github.com/chrisbanes/cheesesquare.