This list will contain sites, divided up into several categories that the average electronics hobbyist would find useful. I would appreciate suggestions on sites people have used that they found useful. This list will not include stores of components suppliers - these are contained within another resource, see here. This list will also not include sites - these can be found in another resource, see here. All the pieces of software in this list will be free. The list will provide the URL for the page on which the download is provided.
The above are only examples, for a full list of EDA see KrisBlueNZ's resource here.
Electronics simulators/schematic viewers Yenka http://www.yenka.com/en/Free_Yenka_home_licences/ Yenka is an extremely easy to use electronics simulator, it comes with an electronics package which includes basic electronics, electronics and electronics with PCB's it also includes maths, chemistry and physics simulators. The options within the simulators are a little limited but it is fine for general use. Unfortunately it can only be used out of school hours to prevent it from being used in school without a paid licence. LTSpice http://www.linear.com/designtools/software/ For information on this see here
Software for hardware Arduino IDE http://arduino.cc/en/Main/Software The IDE for Arduino, required to program an Arduino. The IDE is very easy to use and includes many examples, includes a verify feature to test the software for bugs. Remember it cannot test for hardware bugs! EAGLE CAD http://www.cadsoftusa.com/ An excellent PCB cadcam tool, extremely easy to pick up and an easy to use interface.
3D modelling Sketchup http://www.sketchup.com/ This site contains the downloads for the different versions of Sketchup. Sketchup 8 and Sketchup Make are both free