We've outlined an application that we'd like to build, and attacked the first issues that almost any application raises—session management and (if we want to protect parts of our site from unauthorized users) security.
To do this, we've looked at the CI sessions
class in detail: how it creates session records, and leaves cookies on the visitor's browser. It can then look for cookies when subsequent requests are made. You can use the response to control the way your site responds.
We have also taken a look at some CodeIgniter community libraries. Why don't you go and take a look at the others that there are in the Wiki?
http://codeigniter.com/wiki/Special:Titles
See you in the next chapter!