As you might know, the horde project does not only release a set of production quality software (and an interesting bunch more which are not yet release quality) but also provides over 80 well-designed loosely coupled libraries which help you build websites, business applications or even commandline tools. To stress that point, the Horde Project now put a link to the list of components right on their frontpage. Use Horde_Rdo, a lightweight ORM layer or use the RFC-compliant Imap_Client library which performs equally or even better compared to PHP’s interpreter extension written in c. Horde_Auth, a versatile and pluggable authentication layer, has recently been featured in a series of blog posts by lead developer Jan Schneider.
Like in Symfony or Zend Framework, Components are released along with a PHPUnit based test suite adapted in the Horde_Test class and can be obtained individually through the Horde Pear Channel.