On the PHP::Impact blog, Federico has written up a bit of a "history of PHP applications" since 1998 that have changed the community (and the online world) by being released.

From managing databases to shopping, writing blogs to sending emails. Ten years of passion, great software architectures, team work and revolutionary ideas. Here are the most influential open-source PHP applications to date.

Among those on the list are things like SquirrelMail (1999), phpBB (2000), Drupal (2001) and WordPress (2003). More lately things like frameworks and content management systems have been popular and are being developed with more structure and better standards than some of their predecessors.