A tool developed by the Mozilla project for monitoring a set of builds from a source tree.
It consists of a web-based interface (CGI, written in perl) providing a table containing a column for each different type of build (normally multiple platforms). The top of the table represents the current moment, and as you go down the table, you're going backwards in time. Each individual build (ie, a compile of all source code) is represented by a table cell. When a build is completed, the table cell which represents it is coloured green (if the build is successful), or red (if the build broke).
You can easily view build logs and see the times of checkins (thanks to integration with Bonsai), which makes it easy to track down exactly what actually broke the build, and either get it fixed or reverse the changes out of the tree.
Various open source projects use it, but obviously it's main use is as part of the sophisticated structure of the Mozilla project.