Reduce, Reuse, and Recycle, right? All common, public SDKs and services are fine to use during the hackathon. What's "common"? If someone searching with generic terms can find the service (e.g. [Paris Train Schedule API]) it's fine.
If you have an uncommon/personal open source app or framework you might want to use, e-mail us and we'll list it here so that it's definitely fair game!
Everything listed on the foursquare libraries page