Shiichan is an imageboard that was made for the original world4ch, by a person named Shii. After Shii was fired, the project was minorly carried on by MrVacBob (the current 4chan developer), but was abandoned, especially after world4ch was discontinued.
Is generally disliked and considered to be buggy and full of vulnerabilities.
0ch is the BBS software that was used on the original world2ch and was for a while one of the most popular BBS softwares in Japan (and probably still is). Replaced by 0ch+
0ch+ is a fork of the original 0ch. Used by world2ch and a few others.
A mode that is available within KolymaNET's BBS software Kokonotsuba. Is made to model 0ch and 2ch. Was proprietary for a long time, but is now open-source
Proprietary fork of Futallaby. Used by 4chan.
By far the most popular imageboard software in use today. Is a fork of Tinyboard. Created by Fredrick Brennan, the creator of 8chan and owner of 420chan. Used by many large 'chans such as soyjak.party and leftypol. And was forked countless other times.