There are several possibilities:
Preview.app (comes with OS X) will open any PS (PostScript) file. In the background, I assume, a conversion to PDF takes place, but this should not be an issue.
Adobe Illustrator can open and edit PostScript files. Shelling out a rather big amount of money for just viewing a PS file seems overkill, though. However, there is a free 30-day-trial (link from January 2013 for CS6).
PostView by Metaobject is said to support PostScript as well, but I personally am not convinced / could not get it to work.
My personal recommendation: Use Preview.app!