I've been writing one for a while, but it's not really finished. It's functional at this point, but definitely not easy to set up if you don't know what you're doing.
It does do some cool things though, like automatically generate thumbnails (see my other thread on resizing images) when you uplaod pictures, and lets you give each picture a caption, and put images in categories, use templates for the layout, give each user (supports multiple users) a disk space limit....
I'll help you set it up on your server if you like, I need beta testers
Once it's installed, it's pretty easy to use, I jsut haven't done anything to simplify installation.