The small scribe tool is a must when using royal icing to decorate cookies.  The sharp metal tip allows you to smooth out any gaps or imperfections after filling a cooking with royal icing to help create a smooth finish.  Use perpendicular to your cookie with the tip in the icing and create small circles to even spread your icing.  Works like a charm!

The boo boo stick is a great tool for popping bubbles in icing and many other uses