CodeIgniter was originally developed by Rick Ellis (CEO of EllisLab, Inc.). The framework was written for performance in the real world, with many of the class libraries, helpers, and sub-systems borrowed from the code-base of ExpressionEngine.
It is currently developed and maintained by the ExpressionEngine Development Team.
Bleeding edge development is spearheaded by the handpicked contributors of the Reactor Team.
A hat tip goes to Ruby on Rails for inspiring us to create a PHP framework, and for bringing frameworks into the general consciousness of the web community.