Why not write your own? It wouldn't be too hard:
Have a form that collects a username, email, and comment from the user. Then, write that information into a text file like this:
//this is necessary for correct readback of the file
$comment = eregi_replace("\n","<BR/>",$comment);
$str = "$name|$email|$comment\n";
$file = join("",file("guestbook.dat"));
$fp = fopen("guestbook.dat","w");
fwrite($fp,"$str$file");
fclose($fp);
Then, read back the file like this:
$entries = file("guestbook.dat");
foreach($entries as $entry) {
list($name, $email, $comment) = explode("|",$entry);
//print the data some way here
}
Tada! That's all there is to it!