With NLTK you can build classifiers, decision trees, and train/predict with bayesian classifiers similarly to Google's Prediction API examples. It's pretty easy to get started, and it's code that you run locally, so there is no network traffic.
I use it on http://www.protopub.com for classifying rss feed stories based on user feedback, so Protopub can recommend future stories that you might like. NLTK is far easier than rolling your own classifiers, but even that is not too difficult. See the O'Reilly book Programming Collective Intelligence.