This is known as "Songboard London," known as a "multi-sensory interactive installation" according to its website. The little yellow and black balls allow the public to spin them to change what is being displayed. People have used this to make words and spread messages. Many created typographic messages, which makes this a very interesting example of 3D type. Children and adults alike have used this to make pictures and words. Students at a local university created this outside King's Cross Station, a major transportation hub in London. The installation was up from July to September in 2012, but I believe it should have been permanent. Projects like this allow people to express themselves in a public forum, bringing out the artist in everyone.
Here are links to the official site, and a blog featuring the installation:
http://www.songboardlondon.com
http://collabcubed.com/2012/10/17/song-board-central-saint-martins/
-Franny