WordPress is an amazing tool, I use it on so many websites, I really would be at a loss without it, but what one of the things that makes it so great is the over whelming amount of plugins that are available. If there's something you want to do in WordPress, there's normally a plugin to help you do it.
SEO Ultimate - I think this plugin deserves more recognition than it's gotten up till now. I have never seen a plugin with such a massive array of features, this does everything that platinum SEO and All In One SEO, also throws in the functionality found within SEO Smart Links and a number of other plugins it is essentially the swiss army knife of WordPress plugins. Some examples of its additional features include; code inserter (for use with adding Google Analytics, Adsense section targeting etc) Webmaster verification assistant, Link Mask Generator, Robots.txt editor and there's even an option for exporting your SEO Ultimate settings to a file so you can import later or use on another blog if you need to, along with support for importing post meta data from All In One SEO - and there's more, well worth checking out.
W3 Total Cache Plugin - This plugin when fully configured can give at least 10x improvement in overall site performance and give you up to 80% bandwidth savings via minify and HTTP compression of HTML. In A/B testing this great WordPress Plugin beats other optimisation plugins like WP Super Cache hands down in terms of page load time, and not only that but it has Amazon S3 integration. This plugin is so good at performance optimization that it is trusted by sites such as mashable.com, smashingmagazine.com and many others.
Cbnet Ping Optimizer - Within your WordPress admin panel, if you click on the writing option within the settings tab, you will see at the bottom a section called Update Services, now each time you publish a post or update a post WordPress will ping the sites in this list. So if you update a post frequently, some of these services may blacklist your site, this handy plugin stops WordPress from pinging these services when you edit a post, but allows it to ping them when you first add the post, removing the possibility of being blacklisted as a spammer on those services.
TinyMCE Advanced - WordPress uses the standard version of TinyMCE as its WYSIWYG editor, this is the advanced version that will add 17 plugins to TinyMCE. These include: Advanced HR, Advanced Image, Advanced Link, Advanced List, Context Menu, Emotions (Smilies), Date and Time, IESpell, Layer, Embed Media, Nonbreaking, Print, Search and Replace, Style, Table, Visual Characters and XHTML Extras.
WPtouch - Once this WordPress plugin is installed and activated it will transform your blog into an iPhone application-style theme when it is viewed from an iPhone, iPod touch, Android, Palm Pre, Samsung touch and Blackberry Storm/Torch mobile devices. With web browsing via mobile devices this is an extremely important plugin to utilise, as it will provide fast and user-friendly browsing from the mobile devices mentioned and this will not affect your desktop theme in anyway. On top of that you can customise various aspects of its appearance within the admin panel and it will also display ads from Google Adsense which is very handy.
Maintenance Mode - This nifty plugin will add a splash page to your blog to let its visitors know that your WordPress blog is down for maintenance, it will also inform the visitor when the site will be accessible. The message is completely customisable and logged in administrators will get full access to the blog including the front-end.
Broken Link Checker - It can be quite frustrating for the reader of a page on your blog who has become engrossed in whatever article/post it is they are reading when they try clicking a link within your page and the site is broken. However this will always happen from time to time and can't be avoided, but when you've been blogging for a while and you've amounted a sizable archive of posts and articles it would be impossible to check them all.
WP-DBManager - The worst thing that can happen for any blogger is to put your heart and soul into writing great posts and then then just lose it all, whether it's down to your webhost or something else. This great WordPress plugin makes it so easy to back up your WP database, and even optimise and repair it too. It supports backup scheduling and allows you to either download the backup file or email it to you. This plugin is a must have for anyone who is serious about using WordPress.