In particular I've taken on aspects of community management and code quality in this project - and replaced the existing API with a newer, more robust and flexible version. Joind.in was actually the first open source project I contributed code to; seeing others take their first steps here is an aspect of the project that I find really rewarding.
I'm a project lead on this open source project. The aim of the site is to make it easy to collect feedback from attendees to any event regarding the talks and the event itself. Using this site, conference organisers get feedback, speakers get feedback, and attendees can see what everyone else thought too - and it's all open and happening in real time.