HeeksCAD (free, opensource CAD) for RepRap

HeeksCAD can import solid models from STEP and IGES files, draw construction geometry and lines and arcs, create new primitive solids, or make solids by extruding a sketch or by making a lofted solid between sketches. It can modify solids using blending or Boolean operations. It will save to IGES, STEP and STL, printer plot 2D geometry or to HPGL, import and export dxf files; lines, arcs, ellipses, splines and polylines are supported. Use the geometric constraints solver to create accurate drawings from rough sketches.

Also, it’s a lot easier to start with than something like FreeCAD, which is admittedly a bit more sophisticated but a lot more complicated.

So, check out the wiki and the praise from other users.  But wait, there’s more:

HeeksCNC is an add-on for HeeksCAD. The user can add machining operations, which can then be output to NC code. The NC code is then shown as red and green lines on the screen (below). The NC code is generated using Python script, so it can be configured for different machines.

There’s a wiki for this project, too.