This area is very incomplete. More here to come.
Parametric 3D Modeling Software
Should you not have the money or the budget to do some parametric 3D modeling. Not as easy to use as some CAD packages but it has some great functionality.
3D Printer Control Software
Cura is a software solution for 3D printing. While it is developed to be used with the Ultimaker 3D printer, it can be used with other RepRap based designs. Cura helps in setting up an Ultimaker, shows 3D models, allows for scaling / positioning, can slice the model to G-Code, has editable configuration settings, and send this G-Code to the 3D printer for printing.