Here's a list of some of my favourite SEO tools. If you have some you'd like to share, leave a comment below. Also, take 30 seconds to subscribe to my free newsletter for updates to this and for reviews of new SEO tools.
I'm going to present these SEO tools chronologically according to the steps involved in the SEO process, namely keyword research, competitive analysis, and finally site analysis and progress.
First of all, I want to cover my most-used keyword research tools.
1) Google's Adwords research tool. This is a particularly helpful one because it integrates SEO interest with PPC issues, such as cost and position estimates. My personal preference is to start by looking at what words are getting traffic.
2) Overture's keyword inventory tool. It's got a clean layout, is easy to use, and the results are simple and meaningful. Yahoo has bought out Overture, though I understand results are not from Yahoo's own engine.
3) Hit Tail is a brilliant new product that helps SEOs gain traffic from long-tail searches. Most SEOs, myself included, commit the mistake of only looking at short (one to 3 words) keyphrases. Hit Tail helps suggest longer terms to target. I expect this will make the SEO industry even more competitive.
4) SEOChat's keyword difficulty tool. It provides, at a glance, a rough estimate of how tough a possible keyword selection might be to optimize for. The lower the percentage difficulty returned, the better. Take this with a grain of salt, though, as it isn't some comprehensive algorithm that returns a %100 accurate result.
Second, I want to mention two great tools that modify/adapt the appearance of search results. I use them to research the competition on keywords, which helps determine how much SEO I need to do, and what I should charge.
5) Aaron Wall's SEO extension for Firefox. Displays PageRank, age of site, quantity of links, edu and gov links, del.icio.us and DMOZ links, and more. I appreciate the format and display of the SEO information, in particular, and that it directly links to the source of the info (e.g. Yahoo link: query). It's a free download at Aaron's SEO tools page.
6) Golexa. Similar in nature to Aaron's firefox extension, Golexa displays more information than the SEO extension does. The downside of this is that it's not an extension for Firefox, but is a Google-based engine. This means that you can't use it to find how tough the competition on Yahoo or MSN is. Also, it means a detour to the website, which is an inconvenience. Another issue is that it doesn't display information directly, but requires further clicks. Nevertheless, when I want to get in-depth info, Golexa is handy.
Third, here are a few SEO tools I use to analyze a website and establish its progress.
7) SEOChat's keyword density tool. Again, SEOChat has created a basic, but very useful tool. It returns, with or without meta, alt tags and title tags, the most common words on your website as crawled by a spider. A good indicator your site could use improvement in its keyword density is if, in the first 10 results, there is more than one word of code (such as px, the code abbreviation for pixel).
8) SEOMoz' PageStrength SEO and marketing tool. The beauty of this tool is its holistic approach to marketing that assesses your full marketing efforts. Type in your URL and SEOMoz will let you know how you're doing on Wikipedia, Del.icio.us, Alexa and more than a dozen other factors. It's similar to the extensions mentioned above, but also examines things such as branding, based on how often your site's name is mentioned around the Web.
9) Keyword Position Check tools at Page Rank List and at SEOChat. The first tool isn't as accurate as the second (I think it uses archived results, which means outdated position info on frequently changing keywords), though it provides the benefit of also checking Altavista. SEOChat is more accurate, but only returns results from Google, Yahoo and MSN.
To conclude, a good SEO specialist will include all of these tools in his toolbox, and more. I personally rely heavily on the SEO extension, Google's adwords tool, and SEOChat's tools. Nonetheless, each tool here has a place in the SEO process. Oh, and number 10 ? Try Announce's site submission SEO tool/service, which will suggest you to 30 major engines. Consider also finding out if you've been copied. They say imitation is the sincerest form of flattery...