Jan Koum

Messaging without accounts or fees. Koum, who emigrated from Ukraine as a teenager, co-founded WhatsApp with Brian Acton in 2009, using a phone number as the identity so there was no username or password, and sending messages over data rather than paid texts. A tiny team kept it reliable on cheap phones worldwide, and in 2016 it added end-to-end encryption. Both founders left Facebook, which had bought it in 2014, partly over plans to make money from it.