how to add files to itunes without playing them?

File > Add To Library
Once they've been added, you can uncheck them, so they won't play.

 

iTunes 9 brought a 'watch folder' called "Automatically Add to iTunes", it should be located in your Music folder, dropping files in there will add them to itunes but not play them.

 

+1 on the automatically add to iTunes.

Despite being panned by critics of iTunes as a bad substitute for real folder monitoring (ok, so maybe it is) I still find it very usefull, since it automatically organizes the music into folders in iTunes music.