I like to be organised and have every little detaield planned out as early as possible. My fiancé is basically the opposite. So I found Google Documents really useful for wedding planning.
Click the title of this post to see the "Wedding Template Collection" (or click >here< ) which includes a guest list tracker, address book, to-do list, budget planner, seating chart and day of schedule.
Just a couple of handy documents to keep everything organised. I particularly like the fact that you can publish them to the web and send the link to the people involved (e.g. groom, MOB, bridesmaids?). Very useful!