Please take a look at my Wordpress installation Check list. This blog is written as a follow up to that to suggest some Plugins that I'm using on multiple sites and are becoming essential for some types of website. Some are essential plugins if you're making wordpress website for clients.
Mailchimp for WP Lite - Build yourself a list for sending newsletters.
Jigoshop - Ecommerce solution. There are a heap of these to choose from and some close runners. The thing I like about with Jigoshop is that the logic of code of this plugin seems to make the most sense to me. So it's become my favourite.
WP Help - add help pages to the admin section. This means any help given to clients can just be added to this section. No more looking through emails to find that help that was sent. A must add Plugin for all websites that are not made for your self.
Google Maps v3 Shortcode multiple Markers
My goto plugin to show google maps. Excellent for multiple spots. adding graphics and area shading . Take a look at my blog How to draw multiple circles on your Google Map.
Facebook Comments - let people use their facebook profiles and login to leave comments .
Admin Menu Editor for Wordpress .
Change the look of the Admin menu representation for your users.