Cirrusboard is a minimal and lightweight hobbyist forum software written in PHP. Implementing the core functionality you'd want from a forum, it's perfect for small to medium sized forums and runs lightning fast on anything from dedicated hardware to the tiniest of VPSes.

For more info, see the Cirrusboard Wiki.

This GitHub organisation hosts the Cirrusboard source code, along with repositories with additional themes, smileys and more for Cirrusboard.