Big Help Mob is a project organised by Youth Tree. From the original site, it’s described as:

Big Help Mob is Youth Tree’s latest project currently in development. The idea is simple: Regularly get together 100+ young people from all across the metro area in a big mob and do something useful for a local not-for-profit (tree-planting, renovating homeless shelters etc)… and then celebrate with enormous, ludicrous flash-mobs (100 people looking up at the sky in the CBD for no apparent reason… that sort of thing).

My involvement was the creation of the web application that powers all signups and participation management. It was built from scratch in Rails 3 over the period of a few weeks. More importantly, the whole thing is open source and available on GitHub, encouraging anyone and everyone to contribute.