I like Class::MethodMaker. It's easy to use and it makes quick work of the most irritating part of doing OO, generating accessors and constructors. Class::Struct is nice too, but much more limited in scope and less configurable.

I describe both modules as well as covering several do-it-yourself options in my book. But if you're really interested in learning about all the options you should read this one by Damian Conway.

-sam