I’m looking for a plugin that’ll let me a user filter the images on my gallery page by tag / category. So you click on gallery and it shows you everything; but you can then click on “weddings” or “kittens” to see only images from that category.

Does such a plugin exist?

Yup.

http://weppot.com/gallery-plugin-wordpress/

I use ‘relevant’ on a couple of websites, works well and can embedd on themes etc

http://bestwebsoft.com/plugin/

Cheers chaps, will have a lookie.

The WordPress home page will also give you plenty of examples there’s loads of plugins for it.