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.
Ideally the process would be slightly longer than pressing a button, to increase the feeling of having done something.
So, the London community is arranging a Hackday where some of us will get together and code. In order to ensure we work on the awesomest idea(s) possible, we decided to ask LessWrong to add to our list of candidates. So here is the question:
What could a few developers do in a day or less worth of coding that will be awesome? Also, as a way of checking calibration, you can give your estimate for how long such a thing would take to build.
Note: While we will take ideas and voting here into account, there is no guarantee that we will actually end up choosing one or more of them.