We have:
1. Extensive JIRA integration (and we're working on improving this) https://docs.gitlab.com/ee/project_services/jira.html
2. Jenkins support https://docs.gitlab.com/ee/integration/jenkins.html via our commit status API https://docs.gitlab.com/ee/api/commits.html
3. Slack integration https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/proj... and we're working on a Slack bot https://gitlab.com/gitlab-org/gitlab-ce/issues/20799
We realize that we should allow other tools to integrate well and want to open up as much as we can via API's.