As in, alternatives to programs such as AutoCAD, Solidworks, etc.
There are all-the-way open source ones --- blender, bricad, k3d come to mind. For 2D a number of cad packages are available but naturally their use is very limited. Most are available for various linux distributions. If you're thinking about "free" instead of open source, academic / "non-profit" versions of the "big ones" are an alternative, since I at least haven't really found the open source ones to compete with the commercial ones, unfortunately. Have been trying to work with blender for 3D modeling applications but so far once the problems have become more complicated have "lost it" and started over typically with pro/e.
See http://freecad.com/ There aren't really any major league opensource packages.
Pro-e used to do a free cutdown version called pro-desktop but it sems to have gone.
http://www.intellicad.org/ is a commercial library of cad functions - they used to have a free autocad clone for download.
Blender isn't really a 3d-CAD package in the same way as Pro-E or Inventor it's an excellent 3d renderer.
For simple things there is also google Sketchup - while intended for buildings it wil do other things and is very easy to use.
For simulations look at VTK http://www.vtk.org/ - it's aimed at producing 3d visualisation of your onw data rather than CAD models.
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