Two years ago, I wrote about Improv Everywhere's impromptu food court musical. Since then, they have performed another two in 2009, Grocery Store and I Love Lunch.
More recently (on 24 March), the University of Oregan's all male a cappella ensemble called On the Rocks travelled to New York and whilst on a train in the Subway, 'rick roll'd' other passengers.
Awesome. There is nothing more liberating than bursting into song in public.
Interestingly, there is a huge difference between passengers travelling on the New York Subway and those on the Paris Metro. The New Yorkers in the video appeared to be indifferent. In Paris, they would have been appreciated and given the diversity of busking styles on public transport there, would have received enough Euros for their effort.