But here are the main ones

>> O!m [osu.ppy.sh] [Pretty much inactive]

>> SDVX [imgur.com] [way outdated]

>> Etterna [still playing but mostly offline]

I love high density ordnance
⡆⣐⢕⢕⢕⢕⢕⢕⢕⢕⠅⢗⢕⢕⢕⢕⢕⢕⢕⠕⠕⢕⢕⢕⢕⢕⢕⢕⢕⢕
⢐⢕⢕⢕⢕⢕⣕⢕⢕⠕⠁⢕⢕⢕⢕⢕⢕⢕⢕⠅⡄⢕⢕⢕⢕⢕⢕⢕⢕⢕
⢕⢕⢕⢕⢕⠅⢗⢕⠕⣠⠄⣗⢕⢕⠕⢕⢕⢕⠕⢠⣿⠐⢕⢕⢕⠑⢕⢕⠵⢕
⢕⢕⢕⢕⠁⢜⠕⢁⣴⣿⡇⢓⢕⢵⢐⢕⢕⠕⢁⣾⢿⣧⠑⢕⢕⠄⢑⢕⠅⢕
⢕⢕⠵⢁⠔⢁⣤⣤⣶⣶⣶⡐⣕⢽⠐⢕⠕⣡⣾⣶⣶⣶⣤⡁⢓⢕⠄⢑⢅⢑
⠍⣧⠄⣶⣾⣿⣿⣿⣿⣿⣿⣷⣔⢕⢄⢡⣾⣿⣿⣿⣿⣿⣿⣿⣦⡑⢕⢤⠱⢐
⢠⢕⠅⣾⣿⠋⢿⣿⣿⣿⠉⣿⣿⣷⣦⣶⣽⣿⣿⠈⣿⣿⣿⣿⠏⢹⣷⣷⡅⢐
⣔⢕⢥⢻⣿⡀⠈⠛⠛⠁⢠⣿⣿⣿⣿⣿⣿⣿⣿⡀⠈⠛⠛⠁⠄⣼⣿⣿⡇⢔
⢕⢕⢽⢸⢟⢟⢖⢖⢤⣶⡟⢻⣿⡿⠻⣿⣿⡟⢀⣿⣦⢤⢤⢔⢞⢿⢿⣿⠁⢕
⢕⢕⠅⣐⢕⢕⢕⢕⢕⣿⣿⡄⠛⢀⣦⠈⠛⢁⣼⣿⢗⢕⢕⢕⢕⢕⢕⡏⣘⢕
⢕⢕⠅⢓⣕⣕⣕⣕⣵⣿⣿⣿⣾⣿⣿⣿⣿⣿⣿⣿⣷⣕⢕⢕⢕⢕⡵⢀⢕⢕
⢑⢕⠃⡈⢿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⢃⢕⢕⢕
⣆⢕⠄⢱⣄⠛⢿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⠿⢁⢕⢕⠕⢁
⣿⣦⡀⣿⣿⣷⣶⣬⣍⣛⣛⣛⡛⠿⠿⠿⠛⠛⢛⣛⣉⣭⣤⣂⢜⠕⢑⣡⣴⣿