Is there an app for manage documents for OS X?
The closest analog I know of for general purpose document organizing in an "everything box" similar to iPhoto/iTunes would be Yojimbo.


http://www.barebones.com/products/Yojimbo/


It lets you catalog and organize information in quite powerful ways. Also check out Evernote if you want files to be in the cloud and don't already use a service for syncing. I've heard Together is also worth investigating but never really known anyone that used it or tried it myself.

Finder is capable of using tags and smart searches, but most people that are inclined to do that have already come up with a folder hierarchy that uses the filesystem for storage and spotlight for indexing files. It really depends on how much time you want to invest in cataloging and the volume and type of files you are organizing.