Atlassian Crucible is a collaborative code review application. Like other Atlassian products, Crucible is a Web-based application primarily aimed at enterprise, and certain features that enable peer review of a codebase may be considered enterprise social software. Crucible is particularly tailored to distributed teams, and facilitates asynchronous review and commenting on code. Crucible also integrates with popular source control tools such as Git and Subversion. Crucible is not open source, but customers are allowed to view and modify the code for their own use. (derived from Crucible wikipedia page)
Like Crucible, GitLab provides code review features, and also is optimized to help large (and small) teams work asynchronously. In addition to code review capabilities, GitLab also provides a Git based source code repository, issue tracking and management, CI/CD built-in, security testing, packaging, release, configuration, and monitoring, all within a single application covering the entire DevOps lifecycle.