StayPress - By Clearskys.net. Collection of plugins to turn a WordPress or WordPress MU into a booking system for property rentals, hotel room rentals, conference rooms, B&Bs, training rooms, bands, etc.
WordPress.com Upgrades - Upgrades available at wordpress.com include storage space, using your own domain name (take 'wordpress' out of your url), custom CSS, and unlimited private users (normally limited to 35).