Tom Stoppard wrote a wonderful play called Travesties in which Lenin, James Joyce, and the Dadaist Tristan Tzara all run across each other in Zurich in 1917.

From the Wikipedia page for Vienna's Café Central:

A well known story is that when Victor Adler objected to Count Berchtold, foreign minister of Austria-Hungary, that war would provoke revolution in Russia, even if not in the Habsburg monarchy, he replied: "And who will lead this revolution? Perhaps Mr. Bronstein (Leon Trotsky) sitting over there at the Cafe Central?"

(via @berniemichalik.ca)

A reader fact-checked this: In 1913 Hitler, Stalin, Freud and Trotsky all lived within a 2.36mi (3.8km) radius.

Between January and February of 1913, Stalin took a field trip to Vienna using a forged passport under the name of "Stavros Papadopoulos". He went there at the request of Lenin to investigate the interaction with the most diverse peoples in the multi-ethnic state of the Danube Monarchy. He stayed with a Russian emigrant couple, Alexander Antonovich and Yelena R. Trojanovsky on Schönbrunner Schloßstraße 30.