This probably requires specific knowledge that might make it unsuitable for a group project, but I really like the idea of someone making an Effiicient Charity smart phone app - push a button and some (user defined) amount of change goes directly to your chosen optimal charity (probably with Givewell's top-ranked charity as the default).

Part of the gimmick would be that when you get that little pang of guilt from not giving to someone collecting for charity in the street/beggars/sponsoring your friend to run the marathon, you can easily alleviate it by giving to a useful charity instead.

Unfortunately, I doubt this is the sort of thing on which a small group could produce something awesome in one day, and it probably also requires iOS experience if it's going to get to as wide an audience as possible.