At OSS, Wordpress has become a big part of our production capabilities during the last year. We use it primarily for custom theme development and as a CMS for Flash. Although we don't write many plug-ins, there are a number of third party solutions we can recommend that will enhance any Wordpress powered website. Most of these plug-ins will not be visible on the front-end of your site and won't require any CSS adjustments. These plug-ins are simply our recommendations and are not installed by default on OSS projects unless specifically directed to do so.
Wordpress stores all of its pages, posts, comments and user information in a MySQL database. Backups are usually done manually through software such as phpMyAdmin, normally provided by your web host. Most end-users have no idea how to access phpMyAdmin, much less export and re-import an SQL file. With WP-DB Backup, you can automate SQL file generation at predetermined intervals to either an FTP server or email address. Sending the SQL backup to an email address adds another layer of redundancy should the server become compromised.
http://wordpress.org/extend/plugins/wp-db-backup/
High traffic websites generate thousands of server requests per minute, which negatively impacts page load time and increases strain on the server's CPU. W3 Total Cache compresses HTML, Javascript and CSS pages to reduce bandwidth usage better handle sudden traffic spikes. Faster page loading is increasingly becoming a factor in Google's search ranking. Unlike several other caching plug-ins, you can run W3 Total Cache without modifying your site theme, .htaccess file or server environment.
http://wordpress.org/extend/plugins/w3-total-cache/
Wordpress has pretty robust SEO support natively, but All In One SEO allows you to manually set page titles, canonical URLS and META keywords. All in One SEO also works well with WP e-commerce sites and websites that use Wordpress as a CMS. This plug-in includes many default settings for novices, but advanced users can override them for fine-grained control.
http://wordpress.org/extend/plugins/all-in-one-seo-pack/ - highly configurable SEO tool.
Contact Form 7 is among the most popular form creation tools for Wordpress, due to its support of text fields, drop-down menus, radio buttons, file uploads and effective spam filtering via CAPTCHA.
http://wordpress.org/extend/plugins/contact-form-7/
This is sure to interest graphic designers. All-In-One Cufon allows websites to support desktop based fonts without being forced to utilize Flash or images. Unlike other tools, All-In-One Cufon supports .otf and .ttf files rather than relying on third party font services.
http://wordpress.org/extend/plugins/all-in-one-cufon/