Cruciforum, then.

Cruciforum - a really (really) simple forum, written in PHP.
The goals are pretty explicit:-

A Cruciforum forum is lightweight, doesn’t require a database, and can be set up on your server in about two clicks of a mouse. It’s the easy way to give people somewhere to chat about your project without installing something heavy and complex like PHPBB; just put the cruciforum install file in an empty folder and go to it in your web browser, and that’s all you need to do. Whenever I see a small project’s website I find myself wishing there was somewhere I could talk to people about it, and that’s what this makes easy.

I played with it. I'm impressed. I already started hacking on it a little, fucking about the colours and markup.
So, one thing it could use, is some kind of spam filter thing, and some basic registration system - enter your email address, a password and a given name, maybe. Make the email address optional, even.
But, honestly.. it doesn't really need much more. In fact, it's beautifully simple.
Simplicity really is at the heart of it. The installation makes Wordpress look difficult, which is pretty amazing.

1. Upload the file to your webspace, in a folder, say 'forum'
2. Ensure the folder can be written by the web server
3. Visit the php file in your browser, pick a name for your forum, and you're done.

No databases. No bullshit. And it renders static files, so it's FAST. Like, really fast.
So, some things I'd like...

• I'd like to be able to register, for two reasons.. one, I want to protect my identity, so people can't troll as me, and secondly, filling out my name, URL and email address each time I post (because I want people to know it is me) is a pain.
• Markdown is better than Textile, IMO. Plus, this is just awesome cool.
• Make it really easy for someone to brand this. It doesn't use much in the way of CSS or markup, so do what FogBugz forum lets you do - copy and paste a bit of HTML to appear above your topic list and a bit of HTML to appear below it. Also, offer people a chance to enter a URL to their CSS file and logo if they don't want much.

Go and download Cruciforum now. X11/MIT license.
The boy Langridge done good here.