Hi - You've got some good advice already, but if you want to test one of my favorites, it is the HeadSpace plugin. Can't give a live link yet, but you could Google it, or see the review with screenshots at WebHelperMagazine.com Scott Frangos, Managing Editor - WebHelperMagazine.com
You can post the link here anyway. If people really want to check it out, they'll copy and paste it into their browser of choice.
I've used the Headspace2 seo plugin too and am more than happy with it. I tried out the "all in one" plugin but did find the versatility that I wanted. Headspace takes a bit of setting up, but once you have set up all your preferences it is quick and easy to use on a post by post basis. Lots of options for different keywords and descriptions in different types of posts and pages, and makes tagging easy too. It's the one I settled on.
It is my personal opinion. I wasted some time researching those. Most of those were just link-baits for noobs and their functionality rarely fit your purpose. Average Joe will write/hack/configure needed functionality faster than he'll find the plugin offering the functionality he needs. I mean Sitemap generator is cool, but unfortunately useless as WordPress sites easy to crawl out of the box.
Well, while the sitemap plugins are steaming piles of crap that belong in the sewer system, not all of the plugins are that bad. As I said before, I highly recommend the All In One SEO Pack and Category Base Killer (though I recommend the latter more for the usability than SEO benefits). I just can't wait for the WordPress developers to get off their duffs and incorporate these plugins into the system though (like they are with Search Everything, which has nothing to do with SEO, but allows people to search blog posts as well as WordPress pages).