Simple client for the Have I been pwned? Pwned Passwords API.
Uses range searches and performs all "cryptographic" (hashing) and searching functions in the browser, so nothing except the first 5 characters of the SHA-1 hash of the password is ever sent to the API, maintaining anonymity.
The Web Crypto API's SubtleCrypto functions are theoretically only supported in "modern" browsers, so the site may not work at all in some browsers, although my limited testing indicates that even where support is assumed to be missing the site works fine.