Queryable is free again today. Here's what I posted about it originally:
Queryable is an app that allows you to find a photo in albums using a text description, completely offline.
I tested it out on the 18,000+ photos in my Camera Roll, using the text description "plants on walls." It returned 120 hits, almost all of which were accurate. (It got fooled by a couple of ICM images.) By contrast, using the same text in the native Photos app search returned zero results. For "plants" alone, the native Photos search returned 1700+ hits of plants everywhere. For "walls" alone, it returned 2 hits of our cat posed against a page in a crossword puzzle book.