It’s not exactly lightweight, but Drupal can certainly handle all those things and much more.
I usually recommend Drupal for things like this because it has a good combination of user-friendliness and flexibility. Once Drupal is installed nearly all configuration can be done through a point-and-click Web interface. Only one ‘tech-savvy’ person needs to be involved at all (for installation and adding certain modules) and only sometimes. On the other hand, Drupal’s “hooks” into PHP make it as powerful as a “web framework” like Django or Rails.

Cue framework fanatics flaming for daring to compare their babies to a “mere” CMS.

Take a look at Joomla as well. May suit your needs

I was really looking for something lighter than drupal or joomla.

http://www.narga.net/10-simple-and-lightweight-but-powerful-and-free-cms-solutions/

The two I would investigate are Drupal and Joomla (as previously mentioned). I prefer Drupal because it has a more elegant code base, but it is more technically demanding than Joomla. Joomla also has a very user-friendly administration panel.

It is also possible to create small sites using WordPress, even if you don’t want a blog. Using WP means you can leverage one of the largest support communities for this type of product.

Weebly is also worth a look, but I’m not sure if you can create forums using this tool. Finally, I’ve also used CMS Made Simple, which is great for lightweight sites.

In summary, pick the solution that best suits your needs.

Marty

@martyjacobs Joomla is less technically demanding than Drupal?!
Sounds like I need to give Joomla a second look… the first being several years ago.
I love Drupal personally, but any framework / CMS that is easier to use will likely garner my recommendations from now on.
Is Joomla as flexible as Drupal in your opinion? I regard Drupal as a framework that’s relatively easy to use for non-programmers or beginners (that’s me, even though I’ve been programming for almost 30 years.) As far as I can tell, you can do anything in Drupal that you could do coding plain PHP/[database of choice]. Is Joomla more limited?
I welcome moderation if this comment is off-topic. Readers, flag as you find meet. Mods, if this post is unsuitable, I beg that you PM me that I may know.

@koanhead I feel that both Joomla and Drupal have made HUGE strides as far as user-accessibility in the past two years. I am experimenting with a Joomla installation to suit the book club’s needs as I could not find a suitable mini-cms to fit the bill, and I am quite impressed with the amount of change from 1.5 native to the new 1.6 codebase.

I think Drupal is more extensible, but it also has a larger developer community than Joomla. It is definitely more bare-bones of an install than the other two major CMS choices discussed here (WP and Joomla)

I always thought Joomla was the most cumbersome UI of my big three opensource workflows. That may no longer be the case!

I still love Wordpress over all of the above, but each software has its strengths and weaknesses.