This script is based on Thorn, which in itself makes it bad since Albright (Thorn creator) openly admits to Thorn's code being old and poorly-written. I also recall there being a known SQL exploit for Drydock.
Electronwritten by Taizou[ Perl? ][ 0.7b3 ][ n/a ]
Electron is probably the most obscure old script that I have seen, it is mentioned once on shii.org and once on wakaba.c3.cx, the screenshot I have posted is the only proof I have of its existence.
Futallaby is essentially a translation and slight rewrite of Futaba, it is somewhat popular and Wakaba is partially based upon it, the script is no longer in active development.
I have not tested this script or seen it in actual use, but if the Sourceforge website is anything to go by I would not recommend it for real use. Ignium appears to have an integrated post-voting system of sorts.
This is my favourite script, it has a terrible codebase but it has a tree representation of each individual thread which is composed of avatars that change according to the thread size.
A popular choice for the imageboard community, the codebase is hilariously bad and has several known exploits (patched?). Do not use this script or any variants of it unless you are looking to create drama/receive floods and other goodies from the Internet.
Since I understand this is the only canonical listing of imageboard scripts on the Internet I am going to take this opportunity to say that this software is a crock of shit and the author should stop posting it all over the fucking Internet like it's something more than babby's first PHP script.
Java and Perl-based BBS which allows users to draw on a canvas and post like a normal image. The download link provided is for an older version. (1.32e)