Outclass is a Bayesian classifier for (it relies on POPFile to do most of the actual work, but works seamlessly with in an -only environment). Besides the usual Spam/non-Spam classification, it can also classify e-mail into several "buckets" or categories, and with very little training it soon began to classify my e-mail per project, with no direct mention of the project name or type in the messages - it simply inferred it from its previous training, and without explicitly associating the senders with the project (which you can also do).

Among several neat features, it creates an column in your inbox that shows the category it decided your e-mail belongs to (by which you can group, of course):