Pinterest co-founder Paul Sciarra shared a bit about their stack on Quora:
Python + heavily-modified Django at the application layer
Tornado and (very selectively) node.js as web-servers.
Memcached and membase / redis for object- and logical-caching, respectively.
RabbitMQ as a message queue.
Nginx, HAproxy and Varnish for static-delivery and load-balancing.
Persistent data storage using MySQL.
MrJob on EMR for map-reduce.