Akismet - Where would we be without Akismet? When I first used WordPress Akismet was near enough standard and everybody downloaded it. Now, it is standard and built-in in the latest versions of WordPress. And it is definitely worth using if you are leaving comments open. So get yourself an API key and be sure that that you activate it. If you do not, then you are leaving yourself open to loads of comment spam.
WordPress.com Stats - I first started using this as part of a further plugin, but very speedily grew to find the plugin very useful. In the past I had always relied on Google Analytics for my site stats, but this plugin, which gives less detail, presents what you need to know right in your site. Graphs of up to the minute visitor stats, where they have come from, how they found you and where they went. Invaluable information to improve your site.
WordPress.com Popular Posts - This is the plugin that calls for the stats and why I discovered the stats plugin. This plugin in itself allows you to display a selection of your most often referred to posts. These popular posts are a good way of steering traffic about your blog to areas of interest and a way of keeping them on your website for longer. Ultimately, this builds trust and makes them more prospective to join your RSS or newsletter.
Contact Form 7 - A simple and straightforward to use contact form. It does what it says! But I think it is one of the most useful plugins because it gives you the basis for a contact page, without having to display your email address. If you do display your email, then spammers can scrape it and start filling your inbox. But do not have any contact method available and then no-one might make you advertising offers and so on!
Really Simple CAPTCHA - Again, does what it says! Only one function to this plugin and that is to display a CAPTCHA box on your newsletter. But by using this plugin you are nigh on fully preventing spammers from automatically submitting your contact form and saving yourself heaps of spam!
Yet Another Related Posts Plugin - Where are your readers going to go once they reach the end of the post that they are presently reading? Well, install this little plugin and it will suggest similar themed posts on your website that may interest them. You know what they are reading round now, so offer them more. Again, this is a simple method of increasing the time that visitors spend on your web site and the chances of them becoming a regular reader of your weblog.
Post to Twitter - Nearly perfect this plugin, I use it usually on my websites. It only sends the title of your post to your Twitter account every time you publish a new post, which not just saves you the effort, but gives your posting more exposure via Twitter. This could increase your Twitter followers whilst increasing your site's readers. As I said, almost perfect, I only make a simple change to the way that it posts the title, but this is not really required so use it as it stands.