I use one of these
Cateye flex tight bells. It's tiny and easily detaches when I don't want a bell on my roadbike. It's not very loud but loud enough to alert people on city bike paths and bridges.
The Spurcycle or Crane ENEs are both very nice-just not easy-on easy off like the Cateye. Will probably end up with one someday.
Put a
Incredibell brass duet on my girlfriends beater MTB, that thing is loud! Kinda wish there was a road bike sized model that sounds like that.