User Experience Design
This is a complex question with no simple solution. It depends mainly on two factors:
How much are you willing to pay, if at all?
What do you plan to use it for?
For designing art pieces, vases, and similar gimmicks, you need a polygon based modeler, (think 3dsmax, maya, blender, lightwave, zbrush, modo, mudbox, silo, hexagon etc.) or a NURBS based modeler (Rhino). With the exception of blender which is free (and from my limited experience with it, really weird) all other mentioned apps cost money, and the prices range from $100 to $3000. Each of these has it's advantages and disadvantages which are simply too many to include here. If on the other hand you plan to design functional models, you need some sort of solid (possibly parametric) modeler (inventor, solidworks, spaceclaim) or again use Rhino (which I don't really recommend because it usually creates really bad meshes, often with a lot of errors, at least from my experience). There is also the option of autodesk Fusion 360 which has a free 1 year licence for non-commercial use, and Bonsai3d which could maybe be used for both the aforementioned scenarios and is worth checking out (it also promises some 3d print checking features in the latest version). The main problem with most of these apps is that for good results a lot of practice is necessary. The better the features, the harder it is to learn usually. With the notable exception of SpaceClaim which is really awesome and takes a day to learn. DesignSpark Mechanical is a bit stripped-down free version of SpaceClaim, as already mentioned and should be considered. Openscad is good for people who are more into programming than design and want completely parametric models. For anything else avoid it, you'll be much better of with actual modeling applications. So, there is no single answer to this question. The best advice I can give is to really take your time and check all of these apps to see what features you like, how the interface suits you, and if the price is right for you. Some people will swear by Rhino for example, while I can't stand the sight of it, etc. Get a hold of trial software if possible (it usually is) and see for yourself what fits you best. For any details about specific software and possible usage scenarios, ask, and I'll try to explain the ins and outs as best I can. Here is the list of CAD softwares that are focused mainly on mechanical design and parametric modeling. Although majority of the mechanical CAD market share is dominated by Autodesk, PTC, Dassault and Siemens, there are many other small and medium scale CAD packages which are very useful and economical for small industries. It is important that you invest on the right CAD package. So lets see some of the CAD softwares that are available for mechanical stream. 1.AUTOCAD DESIGN SUITE If you think of mechanical CAD, the first thing that comes in your mind is AUTOCAD. AUTOCAD is developed and distributed by AUTODESK INC. This CAD software is mainly used for creating 2D drawings although it can be used for 3D modeling. In my opinion, 3D modeling is not that user friendly but when it comes to 2D drafting it is the best software available compared to others. 2.AUTODESK INVENTOR Autodesk Inventor is developed and distributed by AUTODESK INC. It directly competes with major 3D CAD modeling softwares like Creo, NX, Catia, Solid works etc. 3.CREO PARAMETRIC Creo parametric formally known as Pro Engineer is developed and marketed by Parametric Technology Corporation (PTC). It is a CAD/CAM/CAE supported software. Pro Engineer was the first mechanical CAD software to bring parametric feature based 3D modeling. Recently Pro Engineer was replaced by Creo parametric series. Easy user interface and powerful modeling options are advantages here. Creo can be easily configured with Windchill (PLM). 4.CATIA Catia is developed and marketed by French company Dassault Systems and has CAD/CAM/CAE support. It is one of the major CAD software in mechanical domain. It directly competes with Creo, NX and Inventor and is particularly used in industries where rich surface modeling is needed such as Aerospace, Automobiles etc. 5.NX NX (UG or Unigraphics) is also one of the major mechanical CAD software which is developed and marketed by Siemens. Being CAD/CAM/CAE software, it is rapidly growing because it has an advantage of easy configuration with Teamcenter (PLM) which is developed by the same company. Teamcenter is already an established PLM software in many industries and domains. 6.SOLIDEDGE Solid Edge is developed and marketed by Siemens. It is the lower end software which only includes CAD. All other major softwares Creo, Catia, NX are CAD/CAM/CAE softwares. It is economical and user friendly compared to NX and used in many small scale industries. 7.SOLIDWORKS Solidworks is a direct competitor of Solid edge. It is economical and user friendly. It is developed by Dassault Systems. It is a major software and used by many companies. 8.DRAFTSIGHT Draftsight is 2D drafting and editing software for DWG and DXF files. It is similar like AutoCAD 2D and is completely free. 9.COBALT Cobalt is a very powerful economical 2D/3D CAD software. It has advanced modeling features and has many modules including surface modeling, Animations and rendering etc.. It has a huge mechanical parts library. It uses parametric modeling. 10.VECTORWORKS Vectorworks fundamentals is 2D/3D mechanical CAD software which is a part of Vectorworks suite. 11.HICAD HiCAD is an other 2D/3D CAD software. It is very economical and has many features in its class. 12.VARICAD VariCAD is another powerful 2D/3D mechanical CAD software which includes crash tests, calculations etc. 13.QCAD QCAD is a 2D drafting software and is economical. 14.SPACECLAIM Spaceclaim is direct 3D modeler software and has many features including geometry repair, geometry editing and CAE support. 15.SKETCHUP Sketcup is developed and marketed by Google INC. It uses pull, move, fill, and combine techniques for modeling and uses basic geometric primitive shapes. Although it is mainly used for layout drawings and it can be used for mechanical design. The basic version is free. 16.PROGECAD PorgeCAD is AutoCAD like 2D/3D software. It is almost similar to AutoCAD with same menus, commands options etc. Its native format is DWG. It is very economical compared to AutoCAD. Its has inbuilt options for PDF to CAD, Image to CAD conversion options. It’s has a huge library of AutoCAD blocks which includes mechanical, electrical, civil etc. 17.BRICSCAD BRICSCAD is a dwg-based 2D/3D CAD software. It has many modules including Assembly modeling, Kinematic analysis, Design Intent Recognition etc. It has all basic necessary tools and options . 18. CADDIEMECHANICAL CADDIE Mechanical is another 2D CAD software. Intelligent markers and mechanical symbols, Tolerances and fits, Balloons and Numbers, Pipes and centre lines, Bolts, nuts and PCD, ISO symbol, diagrams, Sheet metal and tool paths and welding symbols are the features mentioned in their product page. 19.FREECAD FreeCAD is General Public Licensed (GNU GPL) free 3D CAD software. It has parametric modeling options. It can be used for layout drawings, mechanical and architectural 3D modeling purposes. 20.SOLVESPACE SOLVESPACE is a parametric 2D/3D CAD software. It has all basic functions for a 3D CAD. Supports DXF, EPS, PDF, SVG, HPGL, STEP formats. 21.NANOCAD NANOCAD is free 2D drafting software and it has UI and options/features similar like AutoCAD.