Well this is what the title says it is :)
Just a simple guestbook script for use in mIRC.

!guestbook shows the commands
!writegb leaves a message in the guestbook
!viewgb shows the entries.

Have fun with it.
Open up remotes (alt+r) and paste this script in it.

on *:TEXT:!guestbook:#: {
  .timer1 1 1 notice $nick Guestbook v1.2 by ShAmPi
  .timer2 1 2 notice $nick 12-----------------------------------------------------------
  .timer3 1 3 notice $nick 7With this you can leave a message into the guestbook
  .timer4 1 4 notice $nick 7Do NOT abuse this script!
  .timer5 1 5 notice $nick 7The triggers should be typed in the channel the bot is in.
  .timer6 1 6 notice $nick 7The triggers are shown below. Have fun
  .timer7 1 7 notice $nick 12-----------------------------------------------------------
  .timer8 1 8 notice $nick 7The commands are:
  .timer9 1 9 notice $nick 4!writegb <message> - To leave a message in the guestbook
  .timer10 1 9 notice $nick 4!viewgb - to view the guestbook messages
  .timer11 1 11 notice $nick 12-----------------------------------------------------------
}

on *:text:!viewgb:#:{
  set %guestbook.c 1
  /filter -c " $+ $mircdirguest.txt $+ " c:\lines.txt * $+ *
  set %guestbook.total $filtered
  while (%guestbook.c <= %guestbook.total) {
    set %guestbook.msg $read($mircdirguest.txt, %guestbook.c)
    set %gb.nick $gettok(%guestbook.msg,2,32)
    set %gb.time $gettok(%guestbook.msg,1,32)
    set %gb.hour $asctime(%gb.time , HH:nn:ss)
    set %gb.date $asctime(%gb.time , dd/mm/yyyy)
    set %gb.msg $gettok(%guestbook.msg,3-,32)
    .timermsgguestbooknick $+ %guestbook.c $+ $nick 1 $calc( %guestbook.c * 2) .msg $nick 7[ $+ %gb.nick $+ 7]-[ $+ %gb.hour %gb.date $+ ]-7[ $+ %gb.msg $+ 7]
    inc %guestbook.c
  }
}

on *:text:!writegb*:#:{
  if ($2- != $null) {
    /write -a " $+ $mircdirguest.txt $+ " $ctime $nick $2-
    .notice $nick Your message ( $+ $2- $+ )
    .notice $nick is succesfully submitted into the list!
  }
  else {
    .notice $nick Please Fill in Your Message!
  }
}