AdRoll
Based in San Francisco. Online advertising platform, with a specialization in retargeting and real-time bidding.
I work on one of the web applications teams (looking for another senior full-stack developer!), but we are hiring all sorts of Engineers, Designers, Data Scientists, and Product Managers:
<a href="http://www.adroll.com/about/careers/open-positions" >
Stack includes Python, Java, Erlang, JavaScript (Backbone and Angular), R, D, and Big Data platforms all running on lots and lots of AWS.
Ask HN: Who is hiring? (December 2014)