Depending on how light weight you are looking for.. there are tons. I have a huge list you can cycle through here: CMS List or if you can give me more of an idea what you are looking for in a CMS I can try to help.
Wordpress has one of the smallest footprints I have ever seen. Even though its not a "CMS", it can be used as such. There is Tinyportal - http://www.tinyportal.net/ And I think it can be integrated with the simplemachines forum.
A CMS - Content Management System - is merely the skeleton on which you hang your content. In most cases, it consists of HTML/PHP and a database. It's intent is that the CMS handles the framework of your site, and you concentrate on the content that it shows. Wordpress is *certainly* a CMS by every definition of the word ... many CMS's are goal oriented for particular functions - Wordpress is highly optimized for blogging. But just because it's best in a blogging atmosphere does not mean it's no longer a CMS.
I'm looking for something lightweight, and creating a proper, nice code, with CSS used and no tables in the layout, maybe CMS's claim to be "table free" but most of them just give you control over "outside of the content" so you can create your whole layout nicely but once it comes to the article itself it comes in a table, that's my main problem with Joomla so far. Anyway, any lightweight CMS's that create nice code and try to be standard compliant?