Askimet – Checks your comments against the Akismet web service to see if they look like spam or not and lets you review the spam it catches under your blog’s “Comments” admin screen. Best part of Askimet is it comes preinstalled with every version of WordPress. Honestly this is the most valuable plugin you will use. It saves you tons of time from deleting spam posts pointing to viagra and other other ads.

WordPress.com Stats – There are hundreds of plugins and services which can provide statistics about your visitors. However I found that even though something like Google Analytics provides an incredible depth of information, it can be overwhelming and doesn’t really highlight what’s most interesting to me as a writer. That’s why Automattic created its own stats system, to focus on just the most popular metrics a blogger wants to track and provide them in a clear and concise interface.

Cforms II – Is a feature rich plugin for Wordpress, offering convenient deployment of multiple Ajax driven contact forms throughout your blog or even on the same page.

Thank me later – is a plug-in which will automatically send a thank you email to readers after they post a comment. It can be set to send the message a time after the comment was posted (an hour, day, week, month, etc). It is effective as it will remind readers of their comments and gets them engaged with future discussion and your blog. It is useful to attract “one-time” readers who will post one comment, and then forget about your blog.

Google Sitemap – This plugin generates a XML-Sitemap compliant sitemap of your WordPress blog. This format is supported by Ask.com, Google, YAHOO and MSN Search.