Blogs: 

b2evolution  (website)
Nucleus  (website)
pMachine Free  (website)
WordPress  (website)

Portales/CMS:
Drupal  (website)
Geeklog  (website)
Joomla!  (website)
Mambo Open Source  (website)
PHP-Nuke  (website)
phpWCMS  (website)
phpWebSite  (website)
Post-Nuke  (visit site)
Siteframe  (website)
Typo3  (website)
Xoops  (website)

Customer Support:
Crafty Syntax Live Help  (website)
Help Center Live  (website)
osTicket  (website)
PerlDesk  (Commercial, needs license) (website)
PHP Support Tickets  (website)
Support Logic Helpdesk  (website)
Support Services Manager  (website)

Foros:

phpBB2 (website)
SMF  (website)

FAQ:
FAQMasterFlex  (website)

Guestbooks:
ViPER Guestbook  (website)

Hosting Billing:
AccountLab Plus  (Commercial, needs license) (website)
phpCOIN  (website)

Image Galleries:
4images Gallery  (website)
Coppermine Photo Gallery  (website)
Gallery  (website)

Mailing List:
PHPlist  (website)

Polls and Surveys:
Advanced Poll  (website)
phpESP  (website)
PHPSurveyor  (website)

Project Management:
PHProjekt  (website)
dotProject  (website)

Site Builders:
Soholaunch Pro Edition  (website)
Templates Express (website)

Wiki:
PhpWiki  (website)
TikiWiki  (website)

Other Scripts:
Dew-NewPHPLinks  (website)
Moodle  (website)
Noah’s Classifieds  (website)
Open-Realty  (website)
phpAdsNew  (website)
PHPauction  (website)
phpFormGenerator  (website)  
WebCalendar  (website)