A few months ago, I talked to one of the leading CMS industry analysts and he mentioned how surprised he was to find the dominance of PHP in this market.
The available CMSs are anything from open-source freeware to proprietry to supported open-source.
Some familiar names (in no particular order) are Drupal, mambo, eZ and of course, the popular PHP-Nuke.
If I'd go through the whole list of great CMS systems I know in PHP, this post would start getting boring.
As I am asked very often what CMS I recommend, and with all the excellent packages out there it's hard to do so, I decided to post a link to The CMS Matrix which is a nice site that provides some initial matrix comparisons between the various CMS. Of course, at the end of the day you'll have to install some of them and actually try them to see if they suit your needs.