Pronterface is a graphical user interface for desktop 3D printers, such as RepRap. It lets you view Gcode, connect to printer and send the Gcode to it. It's feature rich yet minimalist application.
It allows you not only to send Gcode form file, but also control the printer manually or send Gcode commands directly trough input field.
It integrates with slicing tools, so if you load an STL 3D model to it, it will slice and load it automatically.