Here are some good free programs on the web...

  • PosterRazer: Make your own poster!

    PosteRazor workflow
    The PosteRazor cuts a raster image into pieces which can afterwards be printed out and assembled to a poster. As input, the PosteRazor takes a raster image. The resulting poster is saved as a multipage PDF document. An easy to use, wizard like user interface guides through 5 steps. PosteRazor is available as a Windows, an OSX and a Linux version. It is an open source, GNU licensed project which is hosted on SourceForge.net.