Google is pushing for Phabricator to be used as a tool for LLVM and Clang for handling code review by a wider audience. E-mail will still be the main way of handling code review, but Phabricator is now being pushed as another option.
The LLVM Phabricator announcement came here and the process is documented on LLVM.org.
Phabricator is one of Facebook's open-sourced development tools. Phabricator can be used not only for code review but also for code review but also for tracking bugs, browsing source-code, a Wiki, and much more. Aside from a web-based interface, Phabricator also supports a command-line interface. More information on the project can be found at Phabricator.org.