A great use for open source Hurl is to provide a generic API explorer for API providers. An API provider can install their own Hurl and customize it for their API.
Example API explorers:
What makes a useful API explorer?
Another great explorer: http://gowalla.com/api/explorer
Facebook used to have a really nice API console, but it went away when they introduced the Graph API. Now they have this, which isn't as good but it's something: http://developers.facebook.com/tools/console/
Twitter also has an API console: http://dev.twitter.com/console?content_type=json&path=statuses%2Fhome_timeline