As taken from the CodeIgniter website:
CodeIgniter is a powerful PHP framework with a very small footprint, built for PHP coders who need a simple and elegant toolkit to create full-featured web applications. If you’re a developer who lives in the real world of shared hosting accounts and clients with deadlines, and if you’re tired of ponderously large and thoroughly undocumented frameworks
I must say that after looking through the documentation, watching the introductory screencasts and then experimenting with it myself, it really does live up to it’s claim of real-world thoughtfulness. CodeIgniter flexible and clear when it comes to the MVC pattern, and all through the tutorials I never once felt that I didn’t quite understand what was going on. Even reading the documentation, it’s all amazingly well explained.
I’ve so far not felt mystified by something in the framework, and haven’t yet had to ask any questions on the forums of IRC channels — something I’ve had to do numerous times with CakePHP, a similar open-source project. This of course isn’t to say that Cake is bad, but good documentation is king when it comes to programming.
Overall, the first impression you get when you run CodeIgniter is that of confidence. This is of course the benefit of having a commercial entity backing a project — little things like the documentation that typically don’t get done with a non-commercial project get taken care of.
Best of all? CodeIgniter comes with a license that qualifies as ‘Open Source’.
Lookout CakePHP, you may have the major portion of mindshare right now, but CodeIgniter is a serious contender.