Doppelganger.tk is a site and an Android app that lets you check whether a username is taken across a variety of sites. You can filter for found or not found sites, sort by popularity, filter for a specific site, or only show results for sites you've favorited. It also suggests alternate / past usernames that could be claimed. Username info is cached with a lifetime of 5 hours, so any changes could take up to 5 hours to update.
It uses Sherlock behind-the-scenes to look up usernames. The backend is written in Node.js and hosted on Heroku. The frontend uses Vue.js and is hosted on Firebase hosting. Doppelganger.tk is open source (client server).