An OpenStreetMap "hack weekend" is a meet-up where we bring along laptops to an office space and spend the weekend doing some technical work to improve OpenStreetMap. This may be development of the "core" components, the editors, or any other side projects and pet projects we fancy hacking on. OpenStreetMap has development tasks sprouting from it in all directions. There's work to do in almost any programming language, as well as tasks like documentation, and even some non-technical graphics design and translation tasks.
We mostly take a fairly unstructured free-form format. People turn up and start beavering away on something, or they turn up and see what they can help with.
Official Website: http://wiki.openstreetmap.org/wiki/London/London_Hack_Weekend_Nov_2011
Added by Harry Wood on November 18, 2011