Textportal is a simple script which can be used for a variety of purposes. Put it in a password-protected directory on your site, and it can be used for keeping a private notebook or diary. On a public page, you can use it as a simple chat or shoutbox script, or for sharing images and links.
TinyIB is a lightweight PHP image board which emulates the functionality of 4chan. If you use MySQL or SQLite, you can use it to create an efficient setup able to handle large amounts of traffic. If you don't use a database, it can store posts as text files for a portable setup capable of running on virtually any PHP host.