I’ve got a few sites running on WordPress now and use most of the same plugins. Here are the plugins I consider the ‘base plugins’ for every blog I start:

1. Google XML Sitemaps

XML Sitemap

What it does:
Generates an xml sitemap and submits it to Ask.com, Google, Bing and Yahoo.

Why you should use it:
It’s essential for your SEO that you have a sitemap. This plugin ensures that search engines have up to date data without you having to lift a finger.

Google XML Sitemap by Arne Brachhold

2. All in One SEO Pack

What it does:
Rewrites your URLs to include actual words instead of post numbers, Rewrites page titles, Inserts keywords and page descriptions…

Why you should use it:
To ensure that your website is optimized for search engines. Simple.

All in One SEO Pack by Michael Torbert

3. IntenseDebate

What is does:
Allows advanced commenting options – control the way your comments are displayed and moderated. Intense Debate also allows people to comment using their Facebook or Twitter accounts.

Why you should use it:
I’m more likely to comment on a blog when I don’t have to fill out a big form or register – how about you?

IntenseDebate by IntenseDebate and Automattic

4. Sociable

sociable

What it does:
Sociable displays social networking icons at the bottom of your blog posts/pages allowing users to submit your content to popular websites such as Digg, Facebook, Twitter..

Why you should use it:
Theres a lot of social plugins out there. I use Sociable for 2 simple reasons:
1. You can customise which website icons are displayed.
2. It looks good.

Having links to your website is great for SEO and having your content submitted to social networking websites achieves this.

Sociable By Blogplay

5. Google Analytics

What it does:
Adds Google Analytics tracking code into the footer of your website.

Why you should use it:
Google Analytics is an amazing tool (and it’s free!) that shows you information about who is visiting your site, what browser they’re using, what country they’re in and what they had for breakfast. Well, I think they’re still working on that last one.

In short, you don’t need this plugin if you know how to make changes to your WordPress theme to add Analytics code – but you should definitely be using Google Analytics.

Ultimate Google Analytics by Wilfred van der Deijl.

6. Contact Form 7

What is does:
Allows you to create a page with a form for people to email you. Like this.

Why you should use it:
It creates a nice and easy experience for a person trying to contact you and with Captcha options this plugin also prevents spam.

Contact Form 7 by Takayuki Miyoshi

I use a lot more plugins for specific sites but I consider the above to be my essentials. What do you download as soon as you install WordPress?