Here’s my list of recommended WordPress plugins which I find myself using again and again:
Contact form 7
Very good for doing contact forms. Sends the result to an email address. If you want to save the entries to a DB couple of with “Contact Form DB”.
W3 Total Cache
For site optimisation / speed up. Does a lot of the work for you. Generally enable this when the site is finished.
Fantastic plugin that converts post index page to load posts in an infinite scroll fashion, either automatically or twitter style. Paul Irish heavily involved with this. Uses progressive enhancement so crawlers can still access the posts via traditional pagination links.
Google XML Sitemaps
Automatically generates XML sitemaps for you, submits them to Google.
Adds more security to backend login by adding two-step auth.
Multiple Post Thumbnails
Sometimes you want a post or more often a custom post type to have support for more than one featured image. This plugin allows you to do that.
Add Custom Post Types Archive to Nav Menus
Wordpress doesn’t support this by default but this plugin will allow you to add a custom post type archive page (which usually lists the items) into your menu via the wordpress backend.
Great for traditional style page navigation rather than the simple ‘next post’ / ‘previous post’ links that wordpress provides
Allows you to add ‘tags’ to the media section. Useful if you want as section on your site that lists photos in a particular group.
Very flexible plugin to cover all of your breadcrumb needs.
Social share plugin
Easy way of adding share buttons from several networks under each post.
When using multisite:
WordPress MU Domain Mapping
Allows each site to have it’s own domain which WordPress doesn’t support out of the box.
Allows you to copy / share one post/page to other sites in the network. Works with custom post types too.