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 KIO on KDE (allows accessing ftp, sftp, fish, smb etc.),
    * Printing of differences,
    * Manual alignment of lines,
    * Automatic merging of version control history (cvs Log keyword),
    * and has an intuitive graphical user interface.

WWW: http://kdiff3.sourceforge.net/