Plugins greatly extend the capabilities of a WordPress driven web site. Here is a list of some that are commonly used in many web sites. There are many more than just these, of course. WordPress.Org is a generally a good place to locate plugins. And they are fairly safe to use as the WordPress development team and the WordPress community has vetted them before making them available to the public.
Web Log Tools Collection
WordPress Develoment Community
Dagon Design Form Mailer 5.8
The WordPress plugin version of my secure php form mailer script.
WP_ContactMe is a free WordPress plug-in that provides you with a very configurable contact form. Allowing multiple subject and also SPAM protection.
DESIGN AND INTERFACE
Shutter Reloaded 2.4
Darkens the current page and displays an image on top like Lightbox, Thickbox, etc. However this script is a lot smaller and faster.
WPtouch iPhone Theme 1.9
A plugin which reformats your site with a mobile theme when viewing with an Apple iPhone, Apple iPod touch, Google Android or Blackberry Storm touch mobile device.
WP Shopping Cart 3.8.6
A plugin that provides a WordPress Shopping Cart.
Maintenance Mode 5.4
Adds a splash page to your blog that lets visitors know your blog is down for maintenance. Logged in administrators get full access to the blog including the front-end.
Theme Test Drive 2.8.1
Safely test drive any theme while visitors are using the default one. Includes instant theme preview via thumbnail.
Manages your WordPress database. Allows you to optimize database, repair database, backup database, restore database, delete backup database , drop/empty tables and run selected queries. Supports automatic scheduling of backing up and optimizing of database.
Event Calendar 3.2.beta1
Manage future events as an online calendar. Display upcoming events in a dynamic calendar, on a listings page, or as a list in the sidebar. You can subscribe to the calendar from iCal (OSX) or Sunbird.
Let your visitors share a post/page with others. Supports e-mail and posting to social bookmarking sites.
Twitter Tools 2.4
A complete integration between your WordPress blog and Twitter. Bring your tweets into your blog and pass your blog posts to Twitter.
Adds an AJAX poll system to your WordPress blog. You can easily include a poll into your WordPress’s blog post/page. WP-Polls is extremely customizable via templates and css styles and there are tons of options for you to choose to ensure that WP-Polls runs the way you wanted. It now supports multiple selection of answers.
SEARCH ENGINE OPTIMIZATION
All in One SEO Pack 188.8.131.52
Out-of-the-box SEO for your WordPress blog.
Limit Login Attempts 1.6.2
Blocks an Internet address after a specified limit on retries is reached, making a brute-force attack difficult or impossible.
TAC (Theme Authenticity Checker) 1.4
TAC scans all of your theme files for potentially malicious and unwanted code.
Akismet checks your comments against the Akismet web service to see if they look like spam or not.