I'm a full-stack web developer at Yelp, and I definitely recommend the hell out of working here. We've still got the startup environment (kegs, dog, etc); we're located in downtown San Francisco, half a block from MoMa; we are in a unique position to do great stuff and solve hard problems while helping people find great local businesses.
I built our "Hot New Businesses" feature - http://yelp.com/openings/sf , for instance, you can find it on the homepage and in our mobile apps too - which crunches through our data nightly and generates a list of the most recently opened bars and restaurants in your city, complete with predictions of the day they opened on. It's a really useful feature, and we have so much data that I was able to build it without knowing anything about machine learning, data-mining, etc; imagine the crazy-useful stuff you could do here, if a knucklehead like me could make a feature like this.
☆ Develop cool and useful features for our 61M+ monthly visitors
Search and Data-Mining Engineer
☆ Tackle machine learning and information retrieval problems from our database of 22M+ Yelp reviews
☆ Strong grasp of algorithms and data structures; expertise in Python, Java, or C++
☆ Build whole systems that are simple and scalable
☆ Expertise in your favorite modern programming language: Python, Ruby, Java, Objective-C, or C++ Mobile Developer
☆ Create fun and useful mobile applications for the iPhone, Android, Blackberry platforms and beyond
☆ Expertise in C++, Java and other mobile languages