MiniBBS is a lightweight imageboard and bulletin board system that automatically creates an account for every visitor, thereby bypassing manual registration.
Users have access to their posting history, personal messages, settings, and much more. It uses several effective techniques to prevent spam and other attacks.
MiniBBS is free
MiniBBS is free and open source software released under the terms of the MIT license.
MiniBBS is full-featured
MiniBBS supports tripcodes, image uploading, polls, post editing, searching, tracking of replies to your replies, private messaging, extensive configuration for both users and administrators, and much more.
See the full list of features here.