Extensively Analyze Your Website

If you have a website you maintain on your own, there are all sorts of variables and factors which can fly under your radar. With this Free SEO Analysis tool, you can make sure all the important areas of your site are being maintained and looked after. 

http://www.phpjabbers.com/seo-tool.php

Take Steps To Recover Your Page Rank If It Has Taken a Blow

If you have a site that has a ton of links, there's a good chance you've let a good amount fall between the cracks. When this happens, many times you can generate a large amount of 404 page errors. If you have enough of these you can hurt your rank; turn those errors into 301 redirects with this PageRank Recovery Tool. 

http://www.virante.org/seo-tools/pagerank-recovery-tool

Check Your Search Engine Ranking & Other Important Stats

If you're looking to take your website's search engine ranking a step ahead, then try using this Free SEO Report tool. It provides all the important stats relating to your website's SEO, and other technical readouts that can help with the improvement of your website.
  http://www.seotoolscentral.com/

Keep Track of Web Page Changes with this Monitoring Service

If you have a website you want to keep constant eyes on because of multiple people updating it; or if you want to keep tabs on the competition - try ChangeDetection. This tool will send you automatic updates when a page makes any significant changes. 

http://www.changedetection.com/

Analyze Your Web Page To See If It's 'Search-Engine-Friendly' For Specific Phrases

Simply adding a keyword to a page isn't enough to improve your rankings in search engines. It is also dependent on the placement of the keyword or phrase, and whether you have it in the page's title or not. With this Web Page Analyzer, you can discover whether your page is search friendly or not, then take the necessary steps to make sure it is.

http://www.webmaster-toolkit.com/web-page-analyser.shtml

Analyze Your Content to Identify Missing Relevant Keywords

If you're looking for a way to increase relevancy for your website and in turn increase page rank, then try using the LDA Content Optimizer tool. This tool will crawl your site and crawl similar sites which rank higher. This tool will provide you with keywords missing from your website that are being used by your competition.
http://www.virante.org/seo-tools/lda-content-optimizer

Check the Response Codes of Your Header and Act Accordingly

When a visitor loads your website, all sorts of various responses can load for them. To help diagnose HTTP Header problems, use this HTTP Header Response Tool & Code Key. You can discover what code loads for your website, then see what steps you need to take to fix it.

http://www.internetmarketingninjas.com/header-checker/