Here are a few plugins that can help you with WordPress blog SEO. I could not introduce the post well, as its a quick one from my drafts.

All in One SEO Pack Plugin -
This plugin covers more than 50% needs of SEO for your wordpress blog. It is a full studded plugin with loads of features to play with all your meta tags, of all the pages in the blog which includes the index page, blog post pages, static pages, category and search pages etc.
Customization of Home title, description, keywords, and rewriting of titles, formatting of post and page meta tags, formatting of the 404 pages etc. and more options.

SEO Smart Links -
SEO Smart Links can automatically link keywords and phrases in your posts and comments with corresponding posts, pages, categories and tags on your blog. Further SEO Smart links allows you to set up your own keywords and set of matching URLs. The plugin then processes through the posts and pages and searches for the keywords you want to link to, and automatically links them from the keywords.
The keywords can be searched in through pages, posts, categories, tags. You can limit the number of links per page.
This plugin helps in boosting your rankings through internal linking of particular keywords.

Contextual Related Posts -
Display a list of contextually related posts for the current post. You can select the number of posts to display and if you want to automatically display the related posts in your content / feed. If you want to have similar posts due to similarity in titles or post content, you can select one of them. This can help in building internal links and so rankings are influenced due to this.

SEO Slugs plugin -
Know what Stop words are? Those are like is, and, what, are, on, for etc. words which have no importance in search engines when included in titles of your blog posts. Here, the plugin removes those stop words from the URL of your posts, and the key behind that is -
When the stop words are excluded from the URL, then the keyword density of those keywords included in the post URL increases. This helps in better rankings in search engines.
Suppose if a URL of my post is thewwwblog.com/how-to-perform-due-diligence.html and this will be made into thewwwblog.com/perform-due-diligence.html removing the stop words.

SEO Friendly Images Plugin -
SEO Friendly Images is a WordPress optimization plugin which automatically updates all images with proper ALT and TITLE attributes. If your images do not have ALT and TITLE already set, SEO Friendly Images will add them according the options you set. Additionally this makes the post W3C/xHTML valid as well.
Ranking in image search also helps a lot in gaining traffic to your blogs, and you can rank well when your images have the Alt and Title correctly set with particular keywords you are trying to rank for.
Having images without Alt and Title tags also makes it xHTML invalid. Setting them correctly would make it xHTML valid.
What you need for better SEO is the Alt tag for images which can help you rank better, and title tags are just for your audience.

Redirection plugin -
You are already ranking fine in search engines, and all of a sudden you are making a few changes in your blog or shifting the host. This can make your blog lose the previous URL structure, and so you lose all those visitors who are being set by search engines. The permalink structure which was lost due to the blog changes would still stay alive through this plugin. The plugin helps in redirecting the visitors to the old permalink structure pages which were originally ranking in the search engines.

Google Sitemaps -
A sitemap or “site map” is a single page listing of all the posts on your website. It is intended for your visitors to get a good overview on what your site is about and to find a blog post quickly but it also has great benefits in the search engines as a good link is always pointing to all your blog posts.
The Google sitemaps plugin updates and rebuilds the XML file and then pings the search engines about the new content, which helps in faster indexing of the new content.
Sitemap is always one of the most visited pages by Google as it is always crawled to check for any new content, through this page.

Breadcrumbs Plugin -
This plugin adds breadcrumbs to your theme on every post and page. This helps in 2 ways, firstly it helps in making search engines understand and determine the structure of your blog more easily. And secondly, it helps in better navigation for your readers.
This plugin again can help you in internal linking, and making categories linked more in the posts.