I've had great success with Wordpress. Granted - it's not built to be a CMS per se`, more designed to be a blogging engine, but it does really well with simple plugins, and even custom one's (if you are a little more PHP ambitious).
I actually work for a nonprofit called CURE International, and in the last year me and two other guys (I do HTML, CSS and design, and they do scripting and backend work) have rebuilt our whole site from this really cruddy old CMS all onto Wordpress. Our work even got featured on the Wordpress showcase.
It has proved to be VERY extensible. We're taking advantage of the Blog, the Pages features, User lists for a staff directory, and even built a completely custom plugin for our CUREkids program, which our reps in the field use to upload photos of kids from our actual hospitals in developing countries.
There is even a very slick plugin for ecommerce that I've heard wonderful things about. I'm considering it as one of my top two options for relaunching our CURE store later this year.
To sum it up...
Easy to use and install, both from a server deploy standpoint, and from an admin interface standpoint.
Publishing content is very simple and intuitive.
There is a VERY large developer community, so updates and bug fixes are fairly quick, and lots of plugins and widgets to extend functionality exist.
The CMS is very Google friendly from a markup perspective.
For custom functionality that you need but cannot find in anyone's pluggin or tutorial, modification is pretty easy IF you are comfortable working with PHP
Stackexchange even has a dedicated Q&A site for Wordpress.
Arguably the perfect blogging platform for sites that are essentially a blog-style content site.
Supports multiple authors for content, users, etc.
Even supports multiple sites via Wordpress MU
- Ultimately the system's strength is blogging, so for more extensive functionality, you have to "add on" to the system via plugins, etc. - which means more items to keep track of updating, etc.
Really can't think of any other cons to Wordpress - honestly I'm a big fan, so of course I have way more positive stuff to say than negatives.
Hope this helps!