The Cassandra Project is a modification for the game Deus Ex by Ion Storm. If you don't know what a modification is then I'll tell you:
When a videogame gets released commercially the game's developers sometimes include a set of tools that allow you to create new content using that game as a basis. This can sometimes help to extend the life of the game, because some group of smartarses create a new game that everyone wants to play but they have to go and buy the original game before they can play it. This is how Counterstrike, arguably the most popular online shooter ever, got started. Sometimes it can be way for people who are trying to get jobs in the industry to get noticed by people who want to hire them.
The phrase 'creating new content' can encompass anything from adding a few new bits of textures to the original game, to whole new levels or in extremis cannibalising the entire shooting match to create a 'total conversion' in which the new game bears little to no resemblance to the original engine.
The Cassandra Project was founded by Kieron Gillen on the PC Gamer forum and grew from his original concept of a fast and dirty pop-punk mod to be a vast and sprawling project that would've made the ancient Egyptians think twice about the man-hours.
I joined in as a texture artist half-way through and took on some minor level-building jobs a while later. Eventually the loss of everyone else that had the time or the energy to 'organise' the chaos left me as the organiser, a role somewhat comparable to the chap on the Titanic with the deck chairs.
But hey, you know what? We got something out. While the vast majority of the Deus Ex modding community eventually gave up and lapsed into a graveyard of empty forums and desolate websites full of hollow promises we actually got a realease out, the single most involved briefing sequence in the history of videogames. I kind of like the way that turned out, at the end the mod's main protagonist, Charlotte, drives away to begin her mission and it's full of promise and potential. It's a much better ending than we could have made on purpose. I believe, anyway.
If you own Deus Ex you can get Cassandra Part One, from here.
And Cassandra's official website is here.