Update - 19th July, 2012.
This article is pretty dated now, so you may want to take a look at my most recent article on plugins:
Core plugins for any WordPress site.
Original article starts here:
I follow questions about WordPress on Twitter, and one of the questions that keeps coming up is for recommendations on what plugins to install.
I wrote a post here that explains why that's quite a difficult question to give a useful answer to. However, I've taken the plunge here and set out the plugins I install for new WordPress blogs that I put up.
They're broken down by broad categories:
All-in-one-SEO pack. This allows you to set key META tags for each of your posts and pages - extremely useful. It also allows you to configure how WordPress displays your Post and Blog Titles and set overall META tags for your blog.
SEO Smart Links. This allows you to quickly and easily build the internal linking structure of your blog by identifying posts (and pages) to link together. You can set parameters like how many links you want per post and define specific keywords and posts you'd like the plugin to create links between.
Robots META. This allows you to easily manage robots indexing activities - for example preventing the robots from indexing your admin pages. It allows you to set indexing and no follow, or follow but no indexing on different types of page, giving you a more granular control over the behaviour of the search engine spiders.
Google XML Sitemaps. This will automatically build an XML sitemap of your blog and submit it to the major search engines. It will automatically update it and re-ping the search engines whenever you put up new posts.
Redirection. This will automatically apply a 301 re-direct to any post that has its url changed. You can also set up manual re-directs to posts where you know urls have changed. Particularly useful if you change your permalink structure.
Yet Another Related Posts Plugin. This will display a list of related posts after each article. You can define different weightings for the criteria it uses to identify posts that are related. Great for feeding both readers and the search engines with more juice!
Spam and Security:
Akismet. No link, as this comes as default with WordPress installations. It's been very effective for me at preventing spam.
Bad Behaviour. It works in conjunction with Akismet and does a great job, particularly against screen scraping and link spam.
WP-Security-Scan. This monitors your installation continuously for hacking attempts and blocks them. It also provides you with the facility to easily make changes to your blog configuration to improve overall protection.
Reader Stickiness and Community:
CommentLuv. This attempts to pull in a link from your last post and place it in comments you make on other blogs, providing a deep link back to your blog. If you're a member, you're provided with the opportunity to choose from a selection of posts - useful if you make more than one comment per post on a blog, or more than one comment on the same blog between writing new posts on your blog.
Recently Popular. Displays the most read posts over a time frame you define. Anywhere from a few hours to several months. Shows readers what's hot on your blog at the moment.
Share This. Provides an easy way for readers to share your content with others by bookmarking it or posting it to a selection of bookmarking or social sites. The range of sites that are covered is impressive.
Subscribe to Comments. Enables people to receive email notifications whenever a new comment is added on a post they've already commented on. Brings readers back.
Thank Me Later. Automatically generates a thank you email to people who make comments on your articles. You can write the message that goes out and define how long after they comment that you want to email them. You can set parameters to prevent regular commenters receiving 2 or 3 emails a day. Automatically includes a link to their comment which encourages repeat visits.
Tweet This. Very similar to Share This, but allows people to Tweet your posts.
What Would Seth Godin Do. Allows you to place a message either at the top or bottom of your posts encouraging people to subscribe to your RSS feed. You can actually use it for any purpose at all and you can set it to display a different message to first time readers from what it does for returning readers.
Feedburner Feedsmith. (Because Feedburner is now owned by Google this link will take you to a Google help page - don't be surprised, the link's about a third of the way down). Diverts all outgoing RSS feeds from your site through your Feedburner Feed. This makes them more easily readable to a wider range of RSS readers and gives you detailed statistics on your subscribers.
Cforms11. Enables you to create any type of form you want on your blog. Contact forms, questionnaires, surveys - whatever you want. User friendly, quick to set up, but it definitely helps if you're familiar with CSS.
Search Unleashed. Replaces the standard WordPress Search function (which is abysmal) with something that's much more powerful. Delivers focused and excellent search results.
Administration and Other Stuff:
WP-DBManager. A Database manager that enables you to easily back up, optimise, repair or restore your database.
WordPress.com Stats. A statistics package, which provides some useful views but is limited in many areas. It's good for a quick analysis of your posts over the lifetime of your blog.
Google Analyticator. Adds the code for Google Analytics to your blog, providing a much more detailed analysis of visitors and traffic.
Those are the plugins I generally install on new WordPress blogs. Depending on the subject of your blog there may be others that you should be reviewing.
Remember that plugins add overhead to your site, which can make it slower to load. So you don't want to add plugins that overlap and you don't want plugins that are not providing you with a clear benefit.
So be sure to work out carefully what areas of your blog you want to strengthen and install only those plugins that will bring you a clear benefit. Deactivate and remove anything you're not using to reduce overhad as much as possible.
If you have other plugins you love let us know by leaving a comment!
Updated 6th April, 2009:
Since writing this article I've found and used some more plugins. I've reviewed them here.
Updated 7th May, 2009:
I've seen a lot of questions asking how to install plugins on WordPress blogs, so I wrote a set of step-by-step instructions on how to do so. You can find them here.
Updated 8 July, 2009:
And yet more good plugins - find them here.