Keyword Density Analyzer - Easy SEO
***Favorite - Calculates the percentage that your desired keywords appear in your text. The optimal density for your selected keyword is 2.5%.

Keyword Selector Tool - Overture
***Favorite - Returns a count of the number of times your keyword was searched during the previous month. Use this tool to help you decide which keywords to target.

Reverse Keyword Lookup Tool - URL Trends
***Favorite - This tool shows the keywords and phrases that your competitors hold in Google and where they rank in the search.

Max Keyword Bid Lookup - Overture
***Favorite - Enter a keyword and see the names of companies advertising with your keyword and the maximum amount they are willing to bid. Use this to determine your maximum bid amount when creating your ad campaigns.

Backlink Checker - We Build Pages
Returns a count and a list of sites that link to you. I haven't found a backlink checker yet that returns all backlinks 100% of the time, but this one seems to return the most. Use this tool to see who is linking to you but also to see who is linking to your competitor.

Site Ranking by Keyword - SEO Chat
Shows you who is holding the top positions by keyword. Use this to help you choose a keywords. You may want to try to target some of the keywords that your major competitors aren't pursuing to improve your changes of visibility.

Future Google Page Rank - SEO Chat
This tool will query Google's various data centers to check for any changes in PageRank values for a given URL. Usually all data centers will output the same, but if queried during an update, you might get a glimpse of any upcoming changes in your chosen URL's PageRank value.

Cool SEO Tool - We Build Pages
Returns lots of information about your competitors include: Google, Yahoo, MSN, and Alta Vista rank position, # of backlinks, age of the URL, and # of time the key word or phrase appears on the homepage.

Bad Neighborhood Checker
Straight from Google - "Don't participate in link schemes designed to increase your site's ranking or PageRank. In particular, avoid links to web spammers or "bad neighborhoods" on the web, as your own ranking may be affected adversely by those links."

Plagiarism Finder - Copyscape
Finds potential copies of your copies of your site or pages.

Similar Page Checker
Determines the percentage of similarity between two pages. Google introduced the similar content penalty to reduce page spam. If your page is too similar to another page, Google can and will drop the page that it deems is less important. If you've used reprint articles or use manufacturer product descriptions that other vendors use, add some of your own content as well to avoid a large percentage of duplicate content.

Search Engine Spider Simulator - Webconf
Every page should link be accessible from a path originating on the homepage or search engine spiders won't be able to find and index them. The best way to ensure that all of your pages

.