There are a number of plugins for WordPress that will improve the quality and feature set of any site you want to build. This list includes my ‘must have’ plugins. They get included in every site I build using WordPress.
Flutter I only found Flutter recently, and it is one of the best improvements to WordPress I have ever seen. This is the one to use if you want to completely transform wordpress from a blogging tool to a proper CMS. It allows you to create custom content types with additional fields, image uploads, and more. It allows you to give a far better interface for managing custom fields. It seems to have a few bugs still, and the documentation is horrendous, but if you can figure it out and get it working, it is amazing.
Redirection Very useful if you are migrating from an old site to WordPress. As the name suggests, it redirects old URLs to new ones. It also tracks 404s. My normal process when re-launching a site on WordPress is to map as many URLs as I can, then subscribe to the 404 error log by RSS. If I see pages getting a lot of 404s, I add additional redirects to redirect this traffic.
Plugin Central Lets you install plugins easily from within WordPress. This functionality is becoming available in native WordPress installs, however this one is still useful if you know exactly what you want. This is what I use to install all my other plugins. I can paste my entire list (below) into plugin central, and they are all installed at once. A good time saver. I usually remove or deactivate it once everything is set up.
All in one SEO pack
Google Analyticator
Google XML Sitemaps
Login LockDown
Redirection
Stealth Login
Tracked RSS
Tracked Tweets
WordPress Automatic Upgrade
WP Security Scan
pageMash
Flutter
WP-Devel
Capability Manager
WordPress Automatic Upgrade This has started being phased out by native functionality, but is still useful for the backups and other functionality (maintainence mode, deactivating plugins, etc) it provides. A good timesaver when you have a number of installs to upgrade.
Pagemash Gives an excellent interface for setting page orders and parent/child relationships. Setting page orders in WordPress (particularly in a long list of pages) is almost impossible with the native WordPress functionality.
WP-Devel This plugin adds a toolbar for logged-in users that gives a lot of information about the page you are viewing. Very useful for checking post metadata and wordpress queries while developing the site.
All in One SEO pack Plenty has been written about this one, I don’t think I have done a build without it. I generally don’t touch it too much, just set and forget.
Google Analyticator One of the big Google Analytics plugins for WordPress. This one seems to work, and is updated frequently. It also adds some advanced functionality like outbound click tracking.
Google XML Sitemaps This one helps you get indexed and improve search rankings by telling Google exactly what to do with your site.
Login Lockdown A security plugin that locks people out if they try to access your site more than 3 times with the wrong password.
Stealth Login I use this to move the default login page from yoururl.com/wp-admin to a custom location. This adds an additional hurdle for unauthorised people who want to gain access to your site.
WP Security Scan Another security plugin that tells you all the insecure things about your install, and provides some tools to fix them. I hope this list is useful for you.
The WordPress Plugin Directory is constantly being expanded by thousands of WordPress developers (myself included!), which means that I am constantly trying new plugins. Most of the plugins in this list have been around for a while though, and should be useful in the forseeable future. Have a look at my own plugins if you are interested in tracking Twitter/RSS traffic using Google Analytics.