Ask a founder who their competitors are and you get the three apps they admire. Ask the search results and you get a different list, usually more useful and slightly humbling.
Competitor Finder builds that second list. Give it an app and it pulls the keywords that app ranks on, looks at who else ranks on the same keywords, and returns the ten apps fighting for the same searches, ranked by how much overlap there really is.
It runs on both stores, in any market, without an account.