Psychophysics toolboxes
Roughly put, psychophysics studies the relationships of physical stimuli and their respective elicited sensations and perception. Psyhophysics also relates to the techniques used to probe these relationships and the toolboxes here presented are mainly dealing with these techniques.
OpenSesame is a graphical opensource experiment builder. It has drag and drop features as well as customization possibilities, via python scripting and custom plugins. here is a link to a paper describing the software
Psychtoolbox, or PTB, is a free versatile toolbox to be used mainly in visual experiments, it is able to deliver visual and auditory stimuli and to receive subject input. It has a big quantity of active users (15,000 as stated on their website) what should make the life of the beginner user somehow easier (they have a forum page) The latest version (PTB-3 as this page was written) is able to run under MATLAB (version 7.X) and Octave (version 3.2.X) in any of the three main operational systems out there (Mac, Windows and Linux). A paper describing the toolbox can be found here.
PsychoPy is also a free toolbox that can be used to deliver visual and auditory stimuli and receive inputs from subjects, on top of keyboard, mouse and button boxes, it also supports serial and parallel ports and compiled drivers (allowing interface with pretty much any hardware installed in your computer). It is written in Python, and it can be used with Windows, Mac or Linux. Two papers describing the toolbox can be found here and here.