Review Board is a web-based collaborative code review tool, available as free software

Free software

Free software, software libre or libre software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with restrictions that only ensure that further recipients can also do...

 under the MIT License.

An alternative to Rietveld

Rietveld (Software)

Rietveld is a web-based collaborative code review tool for Subversion written by Guido van Rossum to run on Google's cloud service. Guido based Rietveld on the experience he had writing Mondrian. Mondrian is a proprietary application used internally by Google to review their code.Gerrit is a fork...

 and Gerrit

Gerrit (software)

Gerrit is a free, web-based collaborative code review tool that integrates with Git. It has been developed at Google by Shawn Pearce for the development of the Android project....

, Review Board integrates with Bazaar

Bazaar

A bazaar , Cypriot Greek: pantopoula) is a permanent merchandising area, marketplace, or street of shops where goods and services are exchanged or sold. The term is sometimes also used to refer to the "network of merchants, bankers and craftsmen" who work that area...

, ClearCase, CVS

Concurrent Versions System

The Concurrent Versions System , also known as the Concurrent Versioning System, is a client-server free software revision control system in the field of software development. Version control system software keeps track of all work and all changes in a set of files, and allows several developers ...

, Git

Git (software)

Git is a distributed revision control system with an emphasis on speed. Git was initially designed and developed by Linus Torvalds for Linux kernel development. Every Git working directory is a full-fledged repository with complete history and full revision tracking capabilities, not dependent on...

, Mercurial

Mercurial

Mercurial is a cross-platform, distributed revision control tool for software developers. It is mainly implemented using the Python programming language, but includes a binary diff implementation written in C. It is supported on Windows and Unix-like systems, such as FreeBSD, Mac OS X and Linux...

, Perforce

Perforce

Perforce is a commercial, proprietary, centralized revision control system developed by Perforce Software, Inc.-Architecture:Perforce is a client/server system.The server manages a central database and a master repository of file versions....

, and Subversion.

Review Board can be installed on any server running Apache or lighttpd and is free for both personal and commercial use.

Review requests can be posted manually or automatically using either a REST Web API, or a Python script.
References