I'm going to add a few:
SEO Search Terms Tagging 2 – The purpose of this plugin is to strengthen your On Page
SEO by adding new internal linking to the blog post using the most popular keywords used by search engine visitors to find the blog post from a search engine ( incoming search terms ). After I added this plugin and set it up, my traffic increased by about 70% within three weeks. A must have.
OIO Publisher – If you would like to advertise using banners or links on your blog, this plugin will manage the entire process for you, and automate the process so that buyers can see the prices, order the spot, and even upload any graphics they wish to use, all you need to do is approve the ads and the rest is taken care of for you, and the money is deposited directly into your PayPal account (others are available). It will even take care of turning off the ad after the time has expired so you free up a spot and it becomes available for rent to future advertisers.
WP Super Cache – Now if you’d like to speed up your blog, and avoid any suspensions from your hosting company when your traffic reaches a certain level, or you’ve got some social bookmarking happening then this plugin is for you. It allows you to serve up a cached page and increases the load time of your site. I’ve used a couple of these plugins and prefer the WP Super Cache so far.
All in One SEO Pack - You mention that you installed the
SEO plugin, but I'm not sure which one, this one is usually one of the first ones I install on any blog, and can't say enough good stuff about it, definitely one of my top plugins for sure.
Redirection - Wordpress doesn't handle 301's when you change a slug or move a post for whatever reason, but this little plugin takes care of it, as well as logging your 404 errors so that you can look into and fix them.
Akiskmet - Like Mary said, I'm using this one but the only problem I have is that it does toss legitimate comments in there quite often, which can be a pain to fish them out all the time.
I have to run, but I'll write up a few others I like for you when I get back.
Cheers!