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**][1] 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][2] (link from January 2013 for CS6).

- [**PostView**][3] 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!
 


  [1]: http://www.adobe.com/de/products/illustrator.html
  [2]: http://www.adobe.com/cfusion/tdrc/index.cfm?product=illustrator&loc=en
  [3]: http://www.macupdate.com/app/mac/11601/postview