The PosteRazor cuts a raster image into pieces which can afterwards be printed out and assembled to a poster.
PosteRazor is an open source tool for Windows, OSX and Linux.
http://posterazor.sourceforge.net/
in youtube :
http://www.youtube.com/user/PosteRazor
and her
http://www.youtube.com/watch?v=ukgkWX3cOys