I would suggest you visit
opensourceCMS.com. Take a look at the various options, try out some of them for yourself, usually it's the 'ease of use' of the backend functionality that is the deciding factor. With opensourceCMS, you can try out the software before you install it... so it's well worth a visit.
As for recommendations, I've been told
Mambo is very good, particularly for newcomers to CMS, I'm sure other members will have their own favourites.
Also, if you have your own set of requirements, you may find
The CMS Matrix useful to compare one system against another. You can select up to 10 different CMS's for comparison using the site's matrix.
Paul
PS. I've moved this post from Site Design into Web Programming.