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
foursquare-rails-generator: In 1 minute, get a Rails connected app with Foursquare running on Heroku.
foursquare-app-framework: Connected app template for Python/App Engine
GeoTiles: Geo-points storage for Google App Engine (Python / App Engine)
Snow Shoe Stamp SDKs for iOS and Android.
akdotcom's apps (Python/App Engine)
Singly's sample code:
Groupcentric mobile SDKs: Private sharing and group chat
PassKit4J: Java library for writing Apple Passbook (.pkpass) files
Madison, WI metro data: SMSMyBus