How can you use custom types/taxonomies? in WordPress

Custom types can be leverage for anything that can be repeatable. So as you said a gallery or portfolio would work very nicely. I've already used them on a few different projects:

• Recipes
• Employee Roster
• Callouts for the homepage
• rotating sliders.

Other ideas are:

• products for a commerce site—The custom taxonomies would make for some powerful option for each product.
• For custom sidebar content. Y
• An Ad manager.

The possibilities are really endless. Anything that can be seen as a collection is really what this is for.