OpenSCAD is a software for creating solid 3D CAD models. Unlike most free software for creating 3D models (such as Blender) it does not focus on the artistic aspects of 3D modelling but instead on the CAD aspects. Thus it might be the application you are looking for when you are planning to create 3D models of machine parts but pretty sure is not what you are looking for when you are more interested in creating computer-animated movies.
OpenSCAD is not an interactive modeller. Instead it is something like a 3D-compiler that reads in a script file that describes the object and renders the 3D model from this script file. This gives you (the designer) full control over the modelling process and enables you to easily change any step in the modelling process or make designs that are defined by configurable parameters.
OpenSCAD provides two main modelling techniques: First there is constructive solid geometry (aka CSG) and second there is extrusion of 2D outlines. As data exchange format format for this 2D outlines Autocad DXF files are used. In addition to 2D paths for extrusion it is also possible to read design parameters from DXF files. Besides DXF files OpenSCAD can read and create 3D models in the STL and OFF file formats.
Pronsole is a command line user interface for desktop 3D printers, such as RepRap. It lets you to load Gcode, connect to printer and send the Gcode to it. Best option for controling 3D printer from headless or not enough powerful machines.
It allows you not only to send Gcode form file, but also control the printer manually or send Gcode commands directly to the printer.
Is integrates slicing tool, so if you load an STL 3D model to it as well as Gcode.
Plater is a simple graphical tool for preparing plates for desktop 3D printers, such as RepRap.
It lets you load 3D models and manipulate them on XY plane. It is also possible to let Plater automatically arrange the files to fit. You can than export the prepared plate for later use or load it directly to Pronterface (if installed).
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.
Is integrates slicing tool, so if you load an STL 3D model to it, it will slice and load it automatically.
RepSnapper is an alternative host software for controlling the RepRap open source 3D printer.
OpenGL User Interface (GTK), Slicer and Toolpath Generator written in C++. See also http://reprap.org/wiki/RepSnapper_Manual:Introduction