One of the most annoying things about WordPress is that the default category is always checked when writing a post, so you have to manually uncheck it when you don't want to post in that category.  I never post anything to my default category, so this is especially annoying.  I looked for a solution a while ago, but I couldn't find anything.  Enter Il Filosofo's Uncheck Default Category plugin.  It does exactly what it says by running a little JavaScript on the post-writing screen.  Brilliant, yet simple.