I want to share with you plugins which i use in most of my WP powered sites and hope they will be useful for you. In comments section you can continue the list and write your favourite ones.
Very useful plugin which lets you to write php codes in any post or page.
2. Quick post
Very useful, easy customazible plugin which allows you and other registered users(authors) to post quickly direct from main page sidebar. It might be used if you use blog as question-answer system.