Yesterday Gnip launched! We have a killer service offering. And apparently lots of other people agree. I am totally stoked to be part of the Gnip team.
The Gnip blog does a good job describing why we exist and what we do.
If you are looking for the quick elevator pitch it goes something like this: If are a consumer of public data APIs and would like to stop polling hundreds or thousands of URLs, we provide a way to notify you when any event meeting your interest criteria has happened. If you are a producer of data and would like to allow the world access to your data but the load of thousands of consumers polling your API is hard to manage, we help you by providing data consumers event notification. (Meaning they will only need to hit your API when something they actually care about has happened.)
If either one of those sounds interesting you should definitely check us out.