We use 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.
Oh, and Yashwanth Nelapati would be unhappy if I didn't mention git.
From：http://www.quora.com/Pinterest/What-technologies-were-used-to-make-Pinterest