Object Oriented Bayesian Spam Filtering?

Maybe https://ci-bayes.dev.java.net/ or http://www.cs.cmu.edu/~javabayes/Home/node2.html?

I never played with it either.

I definitely recommend Weka which is an Open Source Data Mining Software written in Java:

Weka is a collection of machine learning algorithms for data mining tasks. The algorithms can either be applied directly to a dataset or called from your own Java code. Weka contains tools for data pre-processing, classification, regression, clustering, association rules, and visualization. It is also well-suited for developing new machine learning schemes.

As mentioned above, it ships with a bunch of different classifiers like SVM, Winnow, C4.5, Naive Bayes (of course) and many more (see the API doc). Note that a lot of classifiers are known to have much better perfomance than Naive Bayes in the field of spam detection or text classification.

Furthermore Weka brings you a very powerful GUI…

Check out Chapter 6 of Programming Collective Intelligence