Classes that aren't models or libraries

[eluser]Hypnotoad[/eluser]
I have some fairly complicated business logic which involves using several different classes. These classes are not models and I don't consider them libraries. Some of these classes may be instantiated many times within different models, so would prefer them to be instantiated manually rather than using $this->load. I'm just not sure where the best place is to store these classes. I was thinking of creating a new directory under system/application. What is considered best practice?

Thanks