Ask HN: Custom search engine you wish existed?

I'm currently experimenting with a couple of text and spatial search technologies, by implementing them on some datasets of interest to myself - arXiv, PubMed, PubChem, OpenStreetMaps are on my radar.

I was wondering what niche search engines you wish existed. If it feels challenging and useful to multiple people, I'll think about implementing it and making it available to everybody. Any other ideas for the datasets mentioned above are also welcome.