CodeIgniter was developed by Rick Ellis, who in his other life is CEO of Ellislab, Inc. The core framework was written specifically for this application, while many of the class libraries, helpers, and sub-systems borrow from the code-base of ExpressionEngine, a Content Management System written by Rick Ellis and Paul Burdick.
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.
The CodeIgniter logo and icons were created by Rick Ellis.
The pull-down table of contents was created with the use of the moo.fx library.