People are always asking me which PHP applications I prefer. It is obviously a very subjective thing, but here is a list of ones I have worked with and liked: FUDForum is a very nice forum package written by Ilia.
Gallery is the ultimate photo album application. You can see it in action at phpics.com
Serendipity is a cool blog package. I really don't like blogs, but as you can see from this toy page which is using Serendipity it doesn't have to be just for your standard boring and useless blog. It can be used for a useless toy page too.