There are so many SEO plugins for WordPress, some better than others of course, some don't do all that much but below are 3 great plugins that I have been using that would be incredibly useful on any WordPress Site.

SEO Ultimate - I think this great plugin deserves more recognition than it's gotten up till now. I have never seen a plugin with such a massive array of features, this does everything that All In One SEO, also throws in the same functionality found within SEO Smart Links and a number of other plugins it is essentially the swiss army knife of WordPress plugins. Some examples of its additional features include; code inserter (for use with adding Google Analytics, Adsense section targeting etc) Webmaster verification assistant, Link Mask Generator, Robots.txt editor and you can even export your SEO Ultimate settings to a file so you can import later or use on another blog if you need to, along with support for importing post meta data from the All In One SEO plugin - and there is more, simply a must have for your blog.

SEO friendly images - It seems like a lot of people will focus more on the off page SEO rather than on page, which itself plays a huge roll in drawing traffic to your site from search engines. Out of all of the on page factors, image ALT & TITLE tags seems to be one of the most underused. This great plugin will automatically update all images with ALT and TITLE tags, the plugin will even add these tags to images that do not have them according to the options you have selected.

Google XML Sitemaps - This is a very useful WordPress plugin, it will create an XML sitemap for your site that will allow it to be indexed better by the major search engines including Google, Bing, Yahoo and Ask.com. Search engines crawlers will be able to use it to see the full structure of your site and notify them whenever any new content is added.

XML sitemaps are very useful as they allow search engines to index pages that are not linked to your site that you wish to be indexed. It tells them the last modified date of pages, how frequently a page is updated so the crawlers know when to check back for new content or updated content and it will allow you to indicate the priority of particular pages within your site.