#1 - JustSearch's Site Comparison Tool
JustSearch's Site Comparison Tool. What better way of getting an indication of how much search engine optimisation your site needs than by comparing your site with one of your competitors. Enter your site and the URL of one of your competitors and you will not only get a decent amount of data but you will also be shown a radar graph of the data. The essentially represents a footprint of your site, compares to your competitor's footprint. Probably the best SEO tool I've seen for a while as it paints a clear picture of what needs to be done.
#2 - Backlink Watch's Back Link Checker Tool
Backlink Watch's Back Link Checker Tool. Most back link tools just tell you how many links you have. This tool goes a step further and looks at the anchor text, PageRank and number of out bound links on the linking page. It will also give you an indication of any abnormal flags like nofollow that have been used on the backlink. It does take some time to run, but it produces a good set of results.
#3 - Dig Pagerank's PageRank Tool
Dig Pagerank's PageRank Tool. Useful to see your PageRank, but more than that. You can view what you PageRank is in lots of different Google data centres and see if a PageRank update is coming your way.
#4 - SearchStatus' Firefox Extension
SearchStatus' Firefox Extension. A must have tool for any SEO that uses Firefox. A simple right click on on the unobtrusive icon will give you access to backlink for either the page or the domain, or even a neat little keyword density checker. The toolbar will also display PageRank, Alexa rank and Compete rank information.
#5 - JustSearch's Nortometer Tool
JustSearch's Nortometer Tool. Odd name, but I've not seen much like this tool out there. The tool looks at the top results in Google for a given keyword and then matches commonly used words a phrases on each of those pages. The result is 1-4 word phrases that you should be including in your content in order to match those sites that rank highly for your keyword. It is a keyword suggestion tool that doesn't use searching statistics.
#6 - Google's Webmaster Tools
Google's Webmaster Tools. The brilliant Google web master tools has a few good functions that are useful to any SEO. With a built in back link checker and a header analyser it is a good way of spotting problems in the long run.
#7 - SEO Chat's Header Checker
SEO Chat's Header Checker. This tool is handy for checking that you have put a correct form of redirect on a page or site. Search engines like 301 redirects the best, so this tool will allow you to see that a 301 is being issued. It is also handy to see that all of your pages are giving 200 codes for normal content as search engines will have issues when they don't see this.
#8 - JustSearch's Keyword Competition Tool
JustSearch's Keyword Competition Tool. This tool takes a keyword as a parameter (with the option of including your own site) and tells you how well your site is doing against the top sites for that keyword. The best bit is that it gives you a radar graph of a section of the results just like the Site Comparison Tool from JustSearch.
#9 - The Internet Archive's Way Back Machine
The Internet Archive's Way Back Machine. When looking at purchasing new domains this tool will help you see how long that domain has been about. Long-standing domains will have more of an impact than brand new ones so seeing that a domain has had a site active on it for a number of years will be beneficial.
#10 - SEO Chat's Robots.txt Generator
SEO Chat's Robots.txt Generator. A nice tool that saves a bit of time when creating a robots.txt file for any site. Just type in a list of directories and files to be excluded and it will generate the file for you.