The following ten tools available without cost would probably be the best for SEO.
1. Tool analyzing the top ten competitors. Available from webuildpages.com it is the easiest method to get a quick glimpse of the ten top most competing on Google for a particular search item.
2. Tool for analysis of search term usage on Web page. This tool could play a crucial role and is probably underutilized by most. It gives an over view of how often an identified search term occurs on a webpage. This free tool is offered by gorank.com
3. The Aftervote tool: A product from youanimous.com. It is a versatile tool, which works across different search engines, and social websites to formulate a picture of inter search engine and social network competition. Therefore it could be termed a Meta SE that collates the results of Google, MSN and Yahoo and blends it with other pertinent factors to give its own results. To the user it gives a snapshot of the sites, which are doing well across these search engines.
4.Tool for Keyword discovery. This tool is helpful to an extent for research on keywords and for ascertaining which keywords are popular. The setback is that it needs more than a months data and sometimes the discovery is made only towards the end of a season. This tool is from inventory.overture.com
5. The Google Toolbar. This tool obviously form Google can be accessed from toolbar.google.com and gives you that immediate view of the value of a webpage. It is indicated by a miniature green bar, which is sometimes cached and needs a pull out with a drop-down. There are other tools, which give way better information, but this is sufficient if all that is needed is a quick look.
6. Spider simulator tool from webconfs.com. One of many hundred available doing the job of giving you an inside view to the links search engines follow and the text as seen by search engines. It is a great way to trouble shoot rather than to be immersed in a heap of code.
7. Tool for Social Book marking aggregation from seo-writer.com. This free tool is more a viral rather than SEO weaponry. Nevertheless as additional people bookmark pages, it adds up to more links being built and results in more bloggers being attracted to pick the bookmark up and then build links the way that search engines like to see them.
8.Tool to identify common links. An ingenious tool from randycullom.com, which leads you to, the common links after identifying the top ten sites from Google and Yahoo for plugged in search term.
9.Yahoo site explorer tool. This shows backlinks to the site or a particular web page and even indexed pages. Providing valuable intelligence on competition it can facilitate waylaying the competitive backlinks to create ones own.
10. Page strength tool from SEOmoz.org. Pulls out information from different indicators and including PageRank from Google and provides a collated view of a value of a page.
I’d add all the tools that Big G himself gave to his humble webmasters: from GWT to keyword investigation tools like GTrends etc.