I'm looking for a very simple forum script such as the forum scripts they use on Ning.com (ex: http://handofwin.ning.com/forum). Where can I get a forum script such as that?

http://www.ohloh.net/p/phpbb
http://www.invisionfree.com

Or, write your own script.

"Write your own" - are you serious??! phpbb3 is probably the best open source solution. The site is down at the moment, but there is a download link. Unfortunately, this means you can't get hold of the all-important mods and styles. Setup is a cinch and admin is straightforward. Translations in many languages. Immensely moddable - see http://startrekguide.com/ for example.

Those are a little more complex, like I said, something simple such as the Ning boards. I want to host it myself as well, so IF would have been out of the question anyway.

If you really want it simple, I like the "write it yourself" approach as well. Paul Hudson has a nice tutorial.

I'm not so fluent in PHP. There's really no other forum out there like the forums used on Ning?