Effective Search Engine Optimization will not be possible without having effective SEO tools. As an SEO consultant I rely on various SEO tools to help me with my keyword research, competitors back links, link popularity, blog pingers, duplicate content checker, keyword density tools, and other SEO and PPC tools.
You can check out SEO tools page of this blog for a very incomplete list of SEO tools I use on day to day bases.
Please forgive me for the lack of organization in that page, I usually just dump a link there once I find it. I will soon organize them and update that page with all the tools I use frequently.
In addition to SEO tools, I have also put list of other SEO and PPC resources such as blogs and other relevant search marketing resources.