Zendesk has put together a simple integration to Campfire so help desk agents can set up support chats with their customers/end-users.
Mikkel Svane, CEO of Zendesk.com, emailed us to let us know:
The integration is pretty basic, but people can expand it any way they want to. Zendesk is also integrated to time tracking service Harvest, and we're currently designing integration paths to both Highrise and Basecamp.

We asked Mikkel for more info and here's what he had to say:
Zendesk is for customer support what Basecamp is for project management. More than 1800 organizations have signed up for the service, and several have asked for a "live chat" feature in our product. Instead, we have used the widget interface that Zendesk provides to build an integration to Campfire.
The sample integration is very basic but Zendesk clients who embed the Javascript widget in their help desk are now able to provide "Live Support Chat" by the flip of a switch. Provided they have a Campfire account. End-users' help desk credentials are automatically transferred to the chat session.
It goes without saying that the widget can easily be extended. If for example you only want to provide live support for registered users, want to publish support conversations in your forums or want to provide 1-on-1 chats.
We ourselves have deployed it in our own help desk to demonstrate the functionality and gather some feedback, and we are considering integrating it directly in our service.
Zendesk widgets are well suited for 3rd party integrations. We have a Harvest integration that is slightly more complex, and integrations are coming to a lot of other services, including Highrise and Basecamp.
Developers: Check out the integration widget code.
The sample integration is very basic but Zendesk clients who embed the Javascript widget in their help desk are now able to provide "Live Support Chat" by the flip of a switch. Provided they have a Campfire account. End-users' help desk credentials are automatically transferred to the chat session.