During spare moments at the Ubuntu Below Zero conference (they're a very friendly bunch of people) I've been hacking on my little spam training tool, written in Mono and Gtk#.
It's been a long time coming, but 0.1.4 fixes a niggling little bug that's been annoying me for a while; in previous releases you couldn't tell if the ham/spam training commands completed successfully. I've added an extra dialog box to alert you if there was a problem. I've also added tooltips to make it more obvious how to train on a message.
You can get it from here: http://spamtrainer.sourceforge.net/