This is a useful guide to the important free SEO tools for doing onsite, keyword and link analysis. As you may have already noticed, there are lots of commercial tools out there that you can use; this article will limit itself to discussing free, quality, reliable tools.
Overview
Bear in mind that there is no such thing as 100% accuracy in using these tools. The important thing is to get practical data from these tools and use it in your analysis. Then your analysis can lead to conclusions and recommendations that can be used to improve your SEO campaign.
You may be familiar with these tools, but lack knowledge of the basic implementation steps or tips on how to use them. Also, remember that this is by no means an exhaustive list of free SEO tools; it will focus only on the important tools and the key features that you can use for doing an analysis.
Keyword Analysis Tools
Before you select any keyword analysis tools, you need to determine your objective in using these tools. Your objectives can tell whether the tool is appropriate and reliable enough to be used. Basically, for any keyword research, you need to go through some general steps.
Step 1. Generate a long list of keyword ideas centering on your targeted niche.
Step 2. Determine whether these keywords are worth pursuing by looking at the estimated search volume figures.
Step 3. Generate some idea of the level of competition of these keywords.
Step 4. Turn the shortened list of keywords into the final, targeted keywords for your campaign.
Of course, after the fourth step, you should re-examine the shortened, listed keywords to make sure that they are directly relevant to your website's content and services, as this can significantly affect your site's conversion rate.
Based on the above steps, the most recommended tool is the Google Keyword tool: https://adwords.google.com/select/KeywordToolExternal. The tool is basically free, and can be used to implement the keyword analysis steps outlined above. The tool can provide a long list of keyword ideas. Checking “Only show ideas closely related to my search terms” can give you a much more specific list of keyword ideas, useful for generating a list of highly related keyword variations.
This is a very versatile tool that can approximate search engine volumes even in different countries, during different seasons, in different languages, on different devices (desktop or mobile searchers), and so forth. It can also provide some idea of the competitiveness of the keyword; just look at the competition column.
If you've already started an SEO campaign and need to track the progress and performance of your targeted keywords, you can use Google Analytics (www.google.com/analytics/ ) and Google Webmaster Tools (www.google.com/webmasters/tools/). You can get tons of useful information from using those tools. Here is a list of some related tutorials that give you further details:
Unlike keyword analysis, onsite analysis needs a more sophisticated variety of tools. This is because of the number of factors that need to be checked during onsite analysis. The following list illustrates this point.
1. Checking the pages indexed in Google – www.google.com using the site command. Example:
site:seochat.com
Entering the above command will provide the full list of pages indexed in Google for the seochat.com domain.
This is basically a crawler that can spot all of the URLs in your website. It can be used for detecting broken links and external links in your website. You can also use the information for generating a customized sitemap. You can read a tutorial on using Xenu sleuth to spot all of the external links in your website: http://www.seochat.com/c/a/Search-Engine-Optimization-Help/Using-Xenu-Link-Sleuth-to-Spot-and-Improve-External-Links-in-a-Domain/. Bad external links (those pointing to spammy and banned websites) can have an effect on your website's reputation.
6. Check to see if your content is crawlable and indexable by search engines – the most important tool to use is a text browser such as Lynx. If you cannot see the text of your website using this tool, then your site has search engine crawling issues. You can read these tutorials for details:
7. Check to see if you have technical search engine-related issues with your website – the most recommended tool is Google Webmaster Tools (GWT): www.google.com/webmasters/tools/
The following are the important issues that can be resolved using GWT:
Crawling errors on your website
Sitemaps
Change of domain address
Setting your geographical target
Find out if malware has been detected on your site
Crawling statistics
Check what Googlebot sees on your website.
Evaluate your site's performance (website loading time).
Reviews 
