Concourse uses Elm for its front-end: https://github.com/concourse/concourse

The elm part: https://github.com/concourse/concourse

https://builtwithelm.co/ lists some, but not sure what you consider large.

Not sure if you would categorize this as dev tool https://github.com/erkal/kite

And ellie certainly might be a dev-tool, but not a toy https://github.com/ellie-app/ellie

https://github.com/rtfeldman/elm-spa-example is a good example app. I'd also recommend https://codebase.show/projects/realworld in general if you're looking for example apps in various frameworks and languages. I sadly can't share anything from work as it's all private.

There's also https://github.com/xbmc/elm-chorus