This plugin hides the default wordpress login screen and makes it easy to integrate logins and such into your theme.
With this plugin you can juice up the default wysiwyg editor. I also tend to activate the following option in it immediately:
"Stop removing the <p> and <br /> tags when saving and show them in the HTML editor"
This plugin provides better pagination in custom templates.
Post Types Order
This plugin helps ordering posts. It is especially useful when dealing with custom post types.