Example of Programming Languages & Frameworks used by YC Companies:

RoR: Tops the prefered framework for the companies in the consumer internet & Enterprise software space. YC startups who are using RoR include: ZenPayroll (Human Resources), Asile50 (retailer deals), BackerKit (Crowdfunding), Rainforest (QA Testing), BuildZoom (Home remodeling marketplace), GinzaMetrics (Sales & Marketing, Advertising) & inDinero (Accounting)

Python Django: The preferred backend language/framework across a wide variety of industries including ZeroCater (Enterprise software connecting restaurants), Casetext (Legal, crowdsourcing), FlightCar (Travel, Shared Services), Ambition (Enterprise software for Sales & Marketing), HightlightCam (Video editing), etc.

Python: Also used in backend scripting, big data & distributed processing, analytics. Notable companies include, Balanced payments, Fivestars (Customer Loyalty Cards), Octopart (Vertical search engine for electronic components), Virool (Mobile Advertising).

C++: Used by a lot of YC companies which require innovations close to the metal/hardware & have high performance requirements. Some companies using C++ include RethinkDB, PipelineDB, for implementing their next generation database, storage technologies. Meta working on their wearable computing project, and Double robotics working on telepresence robot, use C++ as their main language.

Scala: Being used by a stealth company, working on a predictive analytics product. They solve problems related to big data, large storage systems, distributed processing platforms. They prefer to stick with JVM for their platform. Kivo, an document annotations management platform, also used Scala in their software stack.

Golang: Used by Mailgun, in the scaling and load balancing infrastructure. They have published their work in github. Notable projects are vulcan (a HTTP reverse proxy library), gotools-api (to create Restful JSON API), gotools-cassandra (tools to work with cassandra data store). Python is also integral to their software stack.

Haskell: Being used by FOBO, a local marketplace for used goods, in their backend services infrastructure. They seem to have a deep passion for haskell and they seek hardcore generalists with haskell programming skills.

Clojure: Wit.AI is an interesting company, working on artifical intelligence, specifically in the area of understanding natural language & is using Clojure in their NLP (Natural Language Processing) layer. With clojure in their toolbox they should be able to hack and iterate faster on their machine learning algorithms, to meet their ambitious goals.