Galerie is an extremely simple PHP script that I wrote about a while ago.  It's a single file that generates thumbnails.  That's pretty much it.  It's useful if you want something that's extremely stripped down. 
My rating:  3/5 stars

Gallery is huge.  Unbelievably huge.  I tried it and was overwhelmed by its vast array of functions and settings.  It's great if you want something with all the bells and whistles.  It's not good if you're into things like Galerie. 
My rating:  2/5 stars

Zenphoto is the image gallery program/script I've been using on my website for the past several months.  It's got a nice user interface, it's very simple to use, and there's not much to it.  One of the things I like is that I'm able to edit how it works and looks. 
My rating:  5/5 stars

Plogger is pretty cool.  It's similar to zenphoto except that it's much more complicated.  Complicated can be a good thing if you want your photo gallery to do lots of stuff, like commenting (though zenphoto does this), RSS feeds, and packaging downloads.  I personally want something simple. 
My rating:  4/5 stars #technology