SharpCam is a full-featured standalone CAD/CAM (Computer Aided Design/Computer Aided Manufacturing) system used to create NC code for CNC machine tools. Fusion 360 is a 3D CAD, CAM, and CAE tool that connects your entire product development process in a single cloud-based platform that works on both Mac and PC.
The question is a bit vague. Remember that you will need to generate a model in a CAD program (if you are working in 3D), then transform the model to G-Code using a CAM program, then use the G-Code to operate your CNC mill or lathe. For generating your 3D model, I recommend FreeCAD as an open source product. If you want to pay, you can use Inventor, or SolidWorks or many of the other commercial programs. 
I am not sure if this is the proper forum. I am a rookie on SketchUp, and what I want to do is create a cube, similar to a child's block. This would be a cube, with a letter on each face. The letter would be recessed into the surface.
The next step is your CAM program. Unfortunately this step is the weakest in open source programs. You can try PyCAM to generate your tool path if you need a free product. MasterCAM is just too expensive; SprutCAM is a good program and costs just USD1500, but only runs under Windows.
The third step is you machine interface. Mach3 is very good; it costs USD150 and is widely used / pirated. EMC2 is great, it's free and runs under Linux. I use MasterCam everyday, it's versatile and powerful.
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I would recommand it without any doubt. I know delcam also make good product like powermill and featurecam, collegues had used these software and were pleased.
The limitation of Mastercam is that it's not parametric, if you make a change to your model you have to reimport it insede mastercam and reasign the operations to the geometry. If possible try to go with something integrated inside your CAD software like mastercam for solidworks or delcam for solidworks. NX has a built-in cnc porgramming software wich is very powerful I used it at school. Wav file editor for mac. MasterCAM is very expensive and rather buggy. None of the software mentioned in this thread so far is completely free. There aren't very many free options. I will not make a recommendation based on the performance of the software as I have not examined the items on the following list.
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But these softwares are free or open source. MecSoft FreeMILL GNU Computer Aided Manufacturing (sources are on GitHub) HeeksCNC CollabCAD OpenSCAM As Nicolas mentioned before HSMXpress is free for users of SolidWorks who hold a valid license and, honestly, it is the most viable option. The world of free Manufacturing software is an ugly place and considering how the commercial software normally is, it is safer to invest in a well-recommend inexpensive entry-level CAM solution. I also use Mastercam a lot but a novice needs a program with an intuitive interface that he can learn by his own. Imho HSM is one of the best options but is not a standalone program, it is integrated in Inventor or in SW. The most convenient CAD CAM in terms of money is ZW3D 2015 Lite (only 2.5 ops.), it has very nice CAD interface and a potent CAM for 'less than 1000$'. In the other hand we have Vectric Aspire for the woodworking art 1500$ with a lot of tutorials around the web, for cabinets try Cabinet Vision.