Role Manager

Useful if you plan on having multiple users on the blog. You can customize access by user role.

http://www.im-web-gefunden.de/wordpress-plugins/role-manager/

TinyMCE Advanced

This adds Microsoft Word type of editing tools to write your posts. You can customize what buttons you have on there this plugin is golden.

http://wordpress.org/extend/plugins/tinymce-advanced/

Register Plus
This one is strictly for customizing the login page. Nothing more than aesthetics.

http://wordpress.org/extend/plugins/register-plus/

Page Links To

This allows you to create pages that link directly to another site or page directly. Very useful.

http://wordpress.org/extend/plugins/page-links-to/

Avatars

Makes it easy to manage custom avatars on your site. If you plan on using avatars on your site for comments or in other areas, definately check this out.

http://www.sterling-adventures.co.uk/blog/2008/03/01/avatars-plugin/

Contact Form (cforms)

This plugin is very useful. Makes it really easy and simple to manage all form needs for your site whether you have one or twenty.

http://www.deliciousdays.com/cforms-plugin

Flash Video Player
If you plan on having video on your site or posts, check this plugin out. You can download free skins to customize the look and alter what shows on the flash player.

http://wordpress.org/extend/plugins/flash-video-player/

Twitter Tools

Helps you integrate twitter into your site to display your tweets if you wish, or you can choose only to have it post to your twitter account when you post new posts. You can tell it on a post-by-post basis whether you want it to publish a tweet for it. Super handy.

http://wordpress.org/extend/plugins/twitter-tools/

Gallery Plus or Nextgen Gallery

Image gallery plugins each have their own usefulness. You might like Nextgen Gallery more if you want to display different sets or “galleries”.
http://wordpress.org/extend/plugins/gallery-plus/
or
http://wordpress.org/extend/plugins/nextgen-gallery/

WP e-Commerce

If you’re looking for e-commerce to sell things, this plugin is pretty great. It allows for easy management of products, easy shipping calculations, and hooks up natively with paypal or google cart. With a “gold” plugin license (for a small fee), you can connect to other payment gateways, and you can purchase other add-ons.

http://wordpress.org/extend/plugins/wp-e-commerce/