As a frequent contributor to various open source projects, I find that I often want to know just how much I have contributed over the years, and to which projects. With enough time, I could figure out every bug that I've filed, every comment I've posted, every patch that I've submitted (there aren't many), and every contribution I've made. But it would take me a LOT of effort, and after not too long, I'd be knee deep in records and notes of where I had been.
For people that contribute and work on such projects, knowing these kinds of things is valuable in forming an online reputation. This lets people know whether you are a helpful person, what you find interesting, and where your expertise may be. If you're looking for work in such a field, it's great to be able to point to a record of contribution, and say, "Yes, I am interested in this field, and I have a track record to prove it." It creates competition amongst contributors.
But since the current eco-system of online contribution is so diversified, it becomes very challenging to determine a person's online reputation. Some sites do admirable work building in algorithms to calculate the value of users, and this is good. But if you're a person that has been interested in many applications, or that has been working on open-source projects for a long time, it's more likely than not that such systems fall short.
What we need is an aggregated, centralized system that uses public APIs to build global "meta"-reputations. This is likely not that hard, since many of the more-common systems for tracking user contributions already have APIs and RSS feeds for so many things. I'm sure it's more complicated than simply plugging into an API, but creating such a system might not be that hard, and would create great value for the open-source community.