It's essential to the development process that it can be quickly and easily adapted to the changing demands of its market. This is the central idea behind the agile movement and generally good practice. To achieve this agility, we like to work with succinct and expressive programming languages, with Python being our favorite.
Most of our projects are built on Django, as it provides an excellent base for rapid application development due to its extensive library. In API-heavy applications where we don't need Django's content-driven approach, we prefer to use microframeworks such as Flask and Bottle, which are very light and easily extensible.
Our deployment consists mainly of a relational database (usualy PostgreSQL), a non-relational database used for caching or other non-critical/temporary but extremely fast storage needs (usually the amazing Redis), and a fast web server with an equally fast WSGI module (usually nginx with Gunicorn). This is complemented by the fantastic Varnish for caching and load-balancing and, of course, Ubuntu server.