Boorusphere - Yet another booru viewer for Android.

StockSnap - Beautiful, free stock photos

Imgbrd-Grabber - Very customizable imageboard/booru browser and mass downloader, with powerful filenaming features.

Magdeleine - Magdeleine is a stock photos website offering thousands of royalty-free, high-quality photos for personal and commercial projects.

Flexbooru - A booru client for Android, support Danbooru and Moebooru.

Openverse - Openverse is an Openly Licensed Images and Audio database and a growing community of photographers, illustrators, and musicians who share their work under a Creative Commons license.