Well, if you have FSL already installed, FSLview will do it out of the box. I wouldn't install FSL just to look at analyze files, though...

Starting from zero, NeuroLens is probably the easiest (comes as a nice cocoa .dmg file, standard apple installer, relatively small).

Osirix is also a good choice, a slightly more daunting interface to look at first time around, but as easy to install as NeuroLens. It will directly import analyze file and view them, and as a bonus is the best DICOM viewer/importer/anonymizer for the mac that I've found.

And of course all of these are free, and FSL and Osirix are open source (I don't think NeuroLens is, but I could be wrong).

Blaise