Integration with services such as Campfire, Flowdock, Gemnasium, HipChat, PivotalTracker and Slack are available in the from of a Project Service.
You can find these within GitLab in the Services page under Project Settings if you are at least a master on the project.
Project Services are a bit like plugins in that they allow a lot of freedom in adding functionality to GitLab, for example there is also a service that can send an email every time someone pushes new commits.
Because GitLab is open source we can ship with the code and tests for all plugins.
This allows the community to keep the plugins up to date so that they always work in newer GitLab versions.
For an overview of what projects services are available without logging in please see the [project_services directory](https://gitlab.com/gitlab-org/gitlab-ce/tree/master/app/models/project_services).