SEO tools can help automate some of the processes that need to be followed for each of the sites you are optimizing for search engines.
Some of the primary tasks related to the same are:
There are a lot of tools available to accomplish most of the above without breaking a sweat. Here is the list of tools I use, some of them are free and some of them are paid. Some of the tools need to register account and log in to get details about website.
Stats Crop: Stats Crop is a useful tool which is available for free and gives full analysis of website including alexa traffic, website load time, daily pageviews, daily visitors, backlinks, header status, domain age, IP address and server location at one place altogether. All details you will get in a few seconds.
Rage Pank: One of the websites which is providing tool for SEO like keyword research tool, manual spam blocker, and redirect checker. But most important among these, I think, is redirect checker which gives information about the redirect of urls and error code associated with the same.
Go Status: This site needs a signup to use this free tool to get web analytics data of website. This did not look very reliable for very small or relatively new websites. It does however give detailed analysis of data for bigger and relatively older websites.
Alexa: Alexa is a useful tool available for free to get web traffic data, visitors, backlinks. They do also provide a widget with Alexa rank which is based on their own algorithm.
Google Webmaster: Google webmaster tool is the single most important tool which is absolutely free for website owners or webmasters organized by Google provides inbound links, outbound links, content analysis, search queries, site configuration feature, url parameter handling to webmasters. Just only need to setup account for website and put tracking code in your html page of website.
Google Analytics: Most important tool of Google is totally free providing website traffic data including unique visitors, landing page, source, medium, Goal conversion rate, social media impacts, campaigns traffic and e commerce tracking for website. You have to first put tracking code on your html page of website and then you can access all analytics features.
SeoMoz: Website owners can use tool to get web analytics data like duplicate content pages, error pages and their return codes, crawling status, meta tag analysis, redirection errors all kind of stuff you can get from this tool. But tool is not free but paid to get valuable information from this tool.
Majestic SEO: Tool is available for free and provides all historical data about links of a website, keywords and competitors. Only signup for this tool and log in to this website and this will provide all the historical data and for detailed view of web analytics you have to put code on your website provided by this tool. Sitemap Generator? Online sitemap generator tool which saves your time to manually create sitemap for website. Only need to put your url of website and it will create automatically with downloading functionality.
Keyword Spy: Tool is free for users to get highly targeted keywords including organic and PPC keywords and their rankings for their domain, competitors and ads data. Tool is best suitable for large scale websites and not very useful for small or new websites.
Word Tracker: Tool is available for free for 7 days and after that you have to paid for this if you want to keep it up. Very good tool to find keywords for websites provide complete analysis about keywords and competition for keyword.
Google Keyword Tool: Most relevant tool for keyword research of Google adwords which is absolutely free and providing data about keywords related to your query, competition, global and local searches for specific keyword and specific location.
W3C: World Wide Web Consortium is an international community providing functionality about html errors for web pages, broken links, css validation warnings and errors and is absolutely free and trusted to analyze the websites.
This is not a complete list, however, these are the ones that I use regularly and as and when I find any more interesting tools I will keep adding to the list.