CircuitHub (a company I co-founded) and Habito (UK mortgage broker) stand out in my head as startups (in the PG sense of the word http://www.paulgraham.com/growth.html) that use primarily Haskell and have direct startup competition using more conventional stacks.
We come across tons of others, I keep meaning to write down a list.
We come across tons of others, I keep meaning to write down a list.