DataMapper
An Object Relational Mapper written in PHP for CodeIgniter. It is designed to map your Database tables into easy to work with objects, fully aware of the relationships between each other.
Website: http://datamapper.wanwizard.eu/
Gas ORM
A lightweight and easy-to-use ORM for CodeIgniter. Gas was built specifically for CodeIgniter app. It uses CodeIgniter Database packages, a powerful DBAL which support numerous DB drivers. Gas ORM provide a set of methods that will map your database tables and its relationship, into accessible object.
Website: http://gasorm-doc.taufanaditya.com/
Doctrine
Website: http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/
NOTE: You must do some workaround to integrate this with CI, try here.
EDIT: Doctrine, integrating with CodeIgniter (working URL). This might not work for all CodeIgniter versions and may require slight adjustments.
