The best free alternative to osu! is StepMania, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 10 alternatives to osu! and 12 is free so hopefully you can find a suitable replacement. Other interesting free alternatives to osu! are Frets on Fire (Free, Open Source), Clone Hero (Free), McOsu (Free, Open Source) and Flash Flash Revolution (Free).
Clone Hero is a free rhythm game, which can be played with any 5 or 6 button guitar controller, Game Controllers, or just your standard computer keyboard. Download the game, copy your song charts into the song folder and enjoy hours of fun!.
A free open-source circle-clicking rhythm game practice client for osu! beatmaps with Virtual Reality support. McOsu is developed separately from and has no direct affiliation with osu! or ppy Pty Ltd.
Flash Flash Revolution (or FFR) is a music-based rhythm game that runs through Adobe Flash and an Internet browser, making it a full-fledged online music title that requires no downloads or fees to play.
Dance Machine - beat the music brings dance to your mobile! Show that you have rhythm and know how to dance with your fingers. Dance party is coming up! Music run toggle to run the dancers whenever you play music in your device. Its absolutely FREE!