AddOns and packages extend the capabilities of Mathematica. A package is an ASCII file with the suffix .m that contains Mathematica code in a BeginPackage - EndPackage enviroment. This is the standard way to make programs available to others. AddOns are several packages together with HelpBrowser-documentation, palettes and more, all bundeled into a single directory with subdirectories.
AddOns and packages are usually placed into your local AddOns directory. Its location varies from operating system to operating system. Under Windows you can find it under C:/Documents and Settings/All Users/Application Data/Mathematica. On MacOSX it is located under Library/Mathematica. In any case, you can determine the location of your AddOns directory by evaluating the Mathematica system variable $AddOnsDirectory. If you lack writing permissions for that directory use your own subdirectory $UserAddOnsDirectory.
AddOns and packages are placed directly into the Applications subdirectory or any appropriate subfolder therein. Code that should be loaded whenever Mathematica is started can go into the Autoload directory, but it is a better idea to load extensions explicitly. That way users of your code on other platforms will know what is going on and know what there might be missing. To install the HelpBrowser-documentation in an AddOn you have to rebuild the Mathematica Help Browser index by calling up the Mathematica Help menu and activating the Rebuild Help Index menu item. The AddOn documentation is then accessable in the Help Browser under the Add-ons catagory.
To load a package or an addon use the Get (<<) or Needs command. The path and file name are to be given as a Mathematica string with path name seperators indicated by ` (do not confuse accent grave ` with ' or ´). This way your path specifications are system independent! For example, to load the PlotField package from the Graphics subdirectory in the standard Mathematica package library type: <<"Graphics`PlotField`".
The addon developed by us, the biomedical image analysis group at the TU Eindhoven, is MathVisionTools. You can access and dowload this addon by following the menu button to the left. Note, that our addon has undergone two name changes. Previous versions have been called FrontEndVision (or FEV in short) and MathVision.
The menu button Packages to the left leads to a listing of several Mathematica packages, that were written by students, our staff, or others and that we consider useful for image analysis.
Applications are major addon projects, commercial or accademic. The menu button Applications to the left leads to addons that we purchased by our group or that were written by colleages and friends collaborating with us. Here you find the OpenGL interactive 3D viewer MathGL by Dr. Jens-Peer Kuska of Leipzig University, Germany, the QED and QCD package Feyncalc written by Dr. Rolf Mertig, or Digital Image Processing by Prof. Dr. Mariusz Jankowski.