KDiff3 is a program that
    * compares or merges two or three text input files or directories,
    * shows the differences line by line and character by character (!),
    * provides an automatic merge-facility and
    * an integrated editor for comfortable solving of merge-conflicts,
    * supports Unicode, UTF-8 and other codecs, autodetection via 
      byte-order-mark "BOM"
    * supports KIO on KDE (allows accessing ftp, sftp, fish, smb etc.),
    * Printing of differences,
    * Manual alignment of lines,
    * Automatic merging of version control history ($Log$),
    * and has an intuitive graphical user interface.
    * Windows-Explorer integration Diff-Ext-for-KDiff3 - shell extension 
      included in installer (originally by Sergey Zorin: see also Diff Ext)
    * Read what else is special in a short abstract (PDF).