Public Pianos Worldwide
Placing pianos in public spaces has become increasingly popular over the past years, to the point that it's hard to pinpoint who did it first and why. Arguably, the most well-known project is Street Pianos, however this is definitely not the only project. Indeed, private local organisations, city councils, public park authorities, railway companies, and many others have joined in this practice across the world.
The projects are so extensive and localised at the same time, both permanent and temporary, that your question might actually be too broad for Travel SE. The web is scattered with pages on the topic, and maps of pianos. However there is no guarantee that the installations might still be there in the future. It is worthy to note that most outdoor pianos are taken out during the summer and stored away during the rest of the year. Nevertheless, maps are indeed available, therefore I'll summarise my findings nonetheless.
Street Pianos - Worldwide
Street Pianos has been running the Play Me I'm Yours project since 2008, adding and relocating pianos in various cities across the world. Quoting from their website:
Who plays them and how long they remain on the streets is up to each community.
Street Pianos provide a global map of currently available pianos, which can be found here. To date, the project covers 15 cities in the USA.
Piano! Push Play! - Portland, Oregon
Piano! Push Play! is another project, which places pianos in public spaces during the summer, and puts those pianos away in schools and community centres where people can play them throughout the year. The project was born in Portland, Orgeon, and to date this seems to be the only they cover.
The best way to locate the pianos is to use the dedicated app, which shows a map of available instruments. The pianos will also contact you, via the app, when you find yourself close enough to one of them. Alternatively, there is a project webpage detailing all the pianos which you can browse for detailed information.
The Sing for Hope Pianos - New York City
Sing For Hope is a charity who runs a public pianos project in New York City for two weeks during the summer. After which the pianos are donated to local community-based organisation. In 2015, 88 pianos were made available in 5 boroughs across the city. It is likely that the project will run again in 2016, and a map will be rendered available online in due course.
Pianos In Public Spaces - San Diego
The San Diego Symphony rung a public piano project in San Diego. In 2016, from FRIDAY, January 8 – SATURDAY, FEBRUARY 8, 8 public pianos, whose location can be found on the map here, were made available.
And Many More
Finally, if none of the projects above cover your needs, try querying your favourite search engine for
public pianos YYY, where
YYY is the city you will be travelling to, to see if local
YYY organisations have a public piano project running.