Psychtoolbox

Matlab-or Octave-based stimulus presentation library.

+ Multi-platform (Linux, Mac, Windows).

+ Very powerful and with support for numerous hardware peripherals (e.g. eye tracking, data recording).

+ Extremely precise control of timing for recording responses and presenting stimuli (especially visual).

+ Large user community and active forum

+ Numerous well documented demo functions that can be readily modified to suit new experiments

+ Free and open source

- Difficult to learn and requires reasonable command of Matlab/Octave

- No GUI interface

PsychoPy

Python-based stimulus presentation library.

+ Powerful and easy to use with an optional drag-and-drop GUI

+ Large user community

+ Generally good control of timing

+ Well documented

- Some parts are proprietary/commercial