With everyone using those god damn earpods now, I think you could literally have an air horn attached to your bike, and they still wouldn't hear it. I just have a little plastic bell attached to my bike, and I have about a 20% success rate with it. It always gives me pause when someone acts startled, and shocked, yet I've rang my bell 4 times, and my 108 tooth hub freewheeling is literally roaring like a swarm of bees. Yet it's naturally my fault I've startled them.

I use a small and simple bell, mainly because of the size.

Airzound - lighter than a metal bell, 120dB of pure terror. Works against cars. Crane E-Ne for everything else.

Crane Suzu. Looks and sounds nice, reasonably loud, affordable.

130db-150db impact horn with remote.

I've got a little Knog bike bell. You press it once and it lets out a "ding" once. I'll press it two or three times if I see someone wearing headphones who won't hear the "On your LEFT" as I come up on them.

Bonus, I can put my thumb on the bell real easy and it shuts up.

It was cheap, does the job, and is really small.

Air horn because it works against cars.

I just use my lungs. I’m loud from my time in the army so shouting has always worked.

When I was a kid, it was playing card rubberbanded to my chainstay so I could pretend I was riding a motorcycle from the noise

Nowadays, I prefer to just let out wolf whistle. Pierces even the best noise cancelling headphones and busiest traffic noise.

Once I heard a kind verbal "toot toot!" from some toddlers riding past their teeny tiny bikes

Well, my ringtone is the 90's x-men cartoon theme, and it's by far my favorite one ever. Does that count?

I like the horns that go "awoooga."

playing music publicly works and probably the only time it's not annoying.