FSL is a comprehensive library of functional and structural brain image analysis tools, written mainly by members of the Image Analysis Group, FMRIB, Oxford, UK.
FSL is very easy to install and is completely self-contained; you do not need any other software in order to use FSL.
Most of the tools can be run both from the unix command line and as GUIs ("point-and-click" graphical user interfaces). See FSL screenshots and videos.
There is an evaluation and example data package available to accompany FSL.
For up-to-date information regarding FSL see the central version of this FSL home page. For support relating to FSL or related theory, join the FSL email list.
Next - view the list of tools in FSL.