Backlink checkers - verify backlinks to your site, check what anchor text they are using and just generally ensure that your backlink campaign is on track.
Link Analyzer - analyze your inbound and outbound links to make sure all is well, also to be used to that often over looked task - verification of internal links within the site itself.
Keyword density checker - make sure that your pages have the correct keyword density and that you do not look like your are "keyword stuffing" your pages just to get high rankings.
Position checker - make sure that your SEO campaigns are effective and you are climbing up those positions!
URL Submitter - a vital tool to take the leg work out of submitting your site to potentially hundreds of search engines.
Social Web tools - check that links to your site are appearing on the social network sites (Twitter, Facebook etc)
Website Speed Test - there is no point in marketing your site if it does not respond in a timely fashion - visitors will simply not wait!
Source Code tools - CSS validator, HTML validator etc - these may seem like technical tools but poor HTML and CSS can harm your rankings (as well as delivering a poor visitor experience).
Meta Tag Generator - meta tags are important in your ranking efforts and such tools are useful to help make sure you get then right.
Spider Emulator / checker - you should always make sure that your site looks OK when viewed from the perspective of a search engine spider. These tools give you a useful insight in how spiders see your site and help you correct any potential errors.