The Vhost Plugin is my current favorite of the entries for the WordPress Plugin Competition. With this plugin, you can bind categories to virtual hosts - when this host is accessed, the appropriate category is automatically displayed and optionally, a different template can be applied to this combination.
I can think of a whole range of ideas spontaneously - for example, I could put my Bananenrepublik under it and present my rants as more independent content. Or I could finally run my photographic entries under one of my photo domains again. I think I'll play around with the plugin soon.
Another funny feature - but rather uninteresting for me - is the possibility to restrict a user to a VHost (and thus to a category) and set up a multi-blog platform on a WordPress installation. Could be quite funny for family blogs, for example.
Unfortunately, the installation is a bit tricky - you have to patch a few functions because there are bugs in the standard installation. With WP 1.5.1, this should hopefully be better.