PHP Stuffs

phpMyAdmin
Best admin tool for MySQL databases I've found (that's easy-to-use and free, of course.) Every LAMP developer should be familiar with this app by now as you basically just drop it in and go.

phpAdsNew
By far the best Ad Server I have ever come across, free or otherwise. This is an incredibly powerful system that can manage clients, campaigns, publishers/zones, targeting, statistics - pretty much everything you could ever want out of an ad server. Supports pretty much every type of ad - pop-ups, banners, interstitials, flash/shockwave, etc. So wonderful. I always wanted to write something like this, but now I don't have to. Uses MySQL of course and has it's own install script.

phpBB
Pretty ubiquitous software. Has its own templating system, which isn't half bad. The defaults should get you buy pretty well. Anyhow, this is very well-written software. Easy to set up - basically you just drop it in and run the install walk-through. Highly configurable and fairly cleanly written if you want to get under the hood.

Discust
I was looking for something lightweight that had the functionality of a forum/bbs system, but wasn't as gratuitous as phpBB or something of the like. And I wanted threaded (indented) comments a la livejournal, slashdot, etc. This guy wrote a pretty sweet class that's easily modifable and works wonders. Works with a data flat file (no database necessary.)