ChordAI is a sophisticated tool that uses deep learning algorithms to detect chords, beats and even to infer the key of a song. It can accept input from YouTube and from your microphone, and it's got a chord dictionary, audio-to-MIDI conversion and lyrics recognition. That makes it a good tool for musicians and music fans.
Another good option is LyricLab, which helps you write lyrics but also suggests chords based on the key you prefer. It's designed to help people of all skill levels overcome writer's block and write songs that sound good. You can even create parody versions of songs, so it's good for songwriting.
Moises is another powerful tool that can detect chords in real time, along with other features like audio and instrument separation, pitch shifting and a smart metronome. That's good for musicians trying to practice in different tempos or keys, so it's good for drummers, singers, bassists and guitarists.