We (2 people) use php(Reactor) to power a site with almost 40,000 registered users. The majority of the maintenance can be done in our spare time through the administration control panel, the rest of the administration is done by the users of the site. Users can create new forums, become moderators, create polls and list events when they have gained sufficient "status" within the system. When the reactor completes teh warm up cycle, as ours recently did, the content of the site really takes off as the users take over.
The bbs/forums support threaded, flat, and ranked views, double as mailing lists, and make all administration tasks available through the browser interface. One set of forums we run has over 35,000 posts in the database and has shown no signs of slowing down. Our previous store-bought bbs was getting killed when we had only 4,000 posts.
The e-cards are another key application for php(Reactor). I have never found a better way (dollar for dollar) to promote a web site. When we put our cards online our signup rate increased over 300%. The cards system we use supports img, iframe and script cards of any sizes.