For me, the key to a good solid WordPress install is establishing a set of commonly used (and reliable) plugins that can be uploaded with your core WP files so that you can install them right away. In a way you are just creating your own default WP installation that suits your work flow and style of developing.

Here is my short list of essential WordPress plugins that make my life a lot easier:

1. Classy wp_list_pages : Anyone who works with CSS knows that the last and first items in a list are most likely to need special treatment. In WordPress a lot of navigation works off of auto-generated lists of pages and posts hence the need for more control. I wish I had found this plugin a year earlier than I did this is now a must have for my work flow.
2. HITS-IE6 PNG Fix : A fairly comprehensive plugin that provides you with 5 different PNG fix methods for Internet Explorer 6. As PNGs become more and more popular—as well as being preferred by designers who love gradients and drop shadows—the need for PNG fixing is obvious. Very simple, reliable plugin that will do the work for you.
3. PHPEnkoder : I’m a big fan of the HIVE Logic Enkoder in both its on-line and desktop (mac only) forms. So when I discovered that someone had developed a plugin based on their Enkoder I knew I’d like it. This plugin takes any ‘mailto:’ link in your posts and scrambles it so that Bots can’t get your email address… which means less spam. (Note: I have seen this plugin conflict with other plugins so make sure you test. Specifically this does not work well with Shopp because it scrambles email addresses that are entered when ordering)
4. Contact Form 7 : Hands down one of the most powerful and convenient plugins you will find for WordPress. Allows you to create PHP based forms that include validation and send the entry as an email to any address you want.

Those are just the four essential plugins I use for most web sites I work on. If you are looking for a good calendar I suggest Event Calendar 3 and if you need to embed flash content (as I did on this site) I suggest you check out ph-embed Flash.

What makes WP all the more effective and useful are the numerous plug-ins available for all sorts of tasks and additional features. Most plug-ins, however, are FREE and offer good to poor (if any) support. I’m not complaining about this because as a user I’m not paying (but hopefully donating) for the work that those programmers are doing.

On the other hand there are some plug-ins that are starting to emerge that cost money, but offer support and interactivity with the developer. The Shopp plug-in for e-commerce capabilities has really impressed me in what it can do, as well as the service and support behind it.

I am completing my first project using Shopp and not only did I find it very easy to use but any and all problems were addressed, answered and even patched by the development team behind Shopp.

With options to integrate USPS and FedEx accounts as well as various payment services Shopp has far more features than I’ve been able to use. Additionally, it will be much easier to provide clients with e-commerce options as Shopp continues to get better and expand its capabilities.