BackLink Watch
This website helps you check the back links your competitor has earned. This is a great discovery if you are planning to challenge the website in Google rankings. This will give you an idea how much back links you have to earn to supercede the website. But remember! This is not a substitution for a good content website it will only work as catalyst.
Word Tracker
The free edition of word tracker also helps a lot in making a sentence for keyword selection. It shows how many people search a particular keyword in Google per month. The website also associates a free chapter wise e-book to seo that has some really useful tips.
PR Checker
This tool will help you to find Google page rank of a website. If you build link from a higher pr website it will boost your site's search engine rankings.
Fire fox NoDo follow Add -on
This is a really useful utility to find if a link has pr juices or not. The context menu gives you NoDo follow option which when selected colors links that are useful for back linking.
Google's SK tool
This tool is equivalent to word tracker and functions in a similar way. It shows you the exact volume of search that has been conducted over Google search engine in a month for a keyword.
Dead-links
This tool helps you determine the broken links of your website. Broken links portray a bad image of your website in your visitors view and confuses search engines as well.
Web CEO
This is a desktop based software available in both free and paid versions. It helps you determine the Keyword Effectiveness Index (KEI) of a keyword. The higher the KEI, the less competition you have to face to settle in a web niche.