Effective SEO strategies require a lot of effort and time. Although in the search engines market exist very advanced tools that cost a lot, there are many free SEO tools which can help the novice and advanced SEO marketer to save valuable time.
Here is a list of free and proven for their effectiveness SEO online instruments:

1) http://www.alexaranking.com . It displays multiple domains instead of one. Therefore, you can have instant traffic results from Alexa Rankings instead of typing and search each time separately.

2) http://www.xml-sitemaps.com . Sitemaps are extremely important for websites because they help search engines crawl and index them. This is a free xml sitemap generator.

3) http://www.123promotion.co.uk/directorymanager . You can track your submissions to various web directories and you can also visit regularly to see new directories added to the list. You just tick the appropriate submission boxes when you submitted your website. It's very easy to use it.

4) http://www.123promotion.co.uk/ppc/index.php. This is a very powerful tool. It displays, based on the Overture and Wordtracker keyword search tools, similar data, including search figures from the previous month. It also adds statistics for average searches per hour, day, week, projected figures for the next 12 months and then also a figure to see how searches may look in 3 years from now.

5) http://www.seochat.com/seo-tools/keyword-density . This keyword density tool is useful for helping webmasters/SEO's achieve their optimum keyword density for a set of key terms/keywords. This tool will analyze your chosen URL and return a table of keyword density values for one, two, or three word key terms.

6) http://www.mcdar.net/KeywordTool/keyWait.asp . This is another Excellent Resource. When you enter the appropriate URL and keyword, it will display Pagerank and Back links pages for the Top 10 websites.

7)http://www.123promotion.co.uk/tools/robotstxtgenerator.php You can create a free robots.txt file with this resource. So, you will be able to direct the search engines to follow the pages structure of your website and also direct the search engines not to follow and crawl specific web pages of your website you don't want to be crawled.
All you have to do is filling the fields and when the robots.txt file is created you upload it to your root of your web server.

8) http://www.nichebot.com . This website displays keyword data using Wordtracker and Google search results. You just enter the keyword and press the button.

9) http://www.webconfs.com/domain-stats.php . You Enter the domain and get: domain age, number of pages indexed, and number of backlinks. The statistics include Alexa Taffic Rank, Age of the domains, Yahoo WebRank, Dmoz listings, count of backlinks and number of pages indexed in Search Engines like Google, Yahoo, Msn etc.
It will help you figure out why some of your competitors are ranking better than you.

10) http://comparesearchengines.dogpile.com . Shows top results from 3 search engines. This tool helps you get all kind of statistics of your competitor's domains.

11) http://www.marketleap.com/verify/default.htm . This verification tool checks to see if your site is in the top three pages of a search engine result for a specific keyword. You enter your URL/Keyword and it displays top 30 for 11 Search Engines.

12) http://www.related-pages.com/adWordsKeywords.aspx. This tool generates a list of possible keyword combinations based on lists of keywords that you provide.
You enter a list of terms, one per line or separated by commas. This is very effective for Google Adwords and Overture.