The following are plugins that I think all WordPress blogs should have to make your blog ideally functional:
Aksimet and Spam Karma 2: Aksimet is the great default spam killer that comes with every installation of WordPress. Spam Karma 2 is an additional essential plugin that not only gives your blog extra security and you more piece of mind, you also have the option of sending yourself a digest of spam stats, or you can just check the stats and modify many settings in the WordPress admin area so you can control more spam the way you need to.
All-in-One SEO Pack or, Add Meta Tags and SEO Title Tags: SEO is essential for your WordPress blog if you want search engines to find your blog.
WordPress Database Backup: You should definitely backup your database as often as possible and this plugin gives you the option to do so by either downloading it your computer, emailing to yourself or saving it on your server, hourly, daily or weekly.
SRG Clean Archives: Perhaps the best automatic archives plugin that organizes all your past posts in a very neat and simple way.
WP-Email and WP-Print: Give your readers options to email your articles to others or to print the article in a printer friendly way. The cool thing about WP-Email is that your also provided with stats on who and when readers email your articles (and which ones). With WP-Print at the end of the printer friendly version of the article is an ordered list of the URLs in the article, the URL of the blog, and the URL of the article itself.
Instant Upgrade: Allows you to instantly upgrade your WordPress installation to the newest version with one click.
Search Pages: As a default, the WordPress search function only searches through your posts when your reader is types in a keyword to search for. The Search Pages plugins allows your readers to also search pages.
Maintenance Mode: It’s good to have a landing page that lets your visitors know that you are updating or doing maintenance on your site. This plugin lets you automatically publish a temporary home page that lets your readers know that you are maintaining or updating your site.
Arkayne Related Articles or any other related articles plugin: The general consensus for smart blogging is to make your readers stay on your site as long as possible. Having an automatically generated list of related articles at the end of each article usually ensures this.
Add This Social Bookmark: You have to have some sort of way for your readers to spread the word about your blog or a given article with as little complication as possible and as much variety. The Add This Social Bookmark widget is a great simple to install and use plugin that takes up very little room at the end of your articles. Your readers have the option of bookmarking this article through over 35 different social networks and bookmarking sites including del.icio.us, facebook, digg, and so forth.
Dagon Design Sitemap Generator: Sitemaps help your page rankings. So, this simple to install, configure and use plugin will prove useful.
Google Analytics or pMetrics: Both plugins are great for tracking blog activity and generating reports so you know who reads your blog and how they read it. pMetrics is a statistics plugin specific for blogging.
Feedburner or other RSS feed subscription service: You should give your reaeders the option to subscribe to your articles via RSS feed or via email.