I've built a couple different simple web apps for interacting with Mastodon using it's REST API and Oauth.
First, I built Tool for Tabulating Thirty Top Tooters, which allows users to find other users they've favorated the most. (source).
Second, I built Mastodon-de-mob, a tool designed to help prevent harassment by letting users block all the users who boosted or favorated a harassing toot. I hope it's never needed, but I'll be glad I built it in advance if it is.