Software designed to provide a count of your chosen keyword/s within the content you specify. Be aware that the amount of html markup will have an impact on your density percentage and keyword prominance.
Software that checks your content for valid hyperlinks and reports any faults it finds. This is vital maintenance for your site in terms of sound SEO practice.
Since the invention of google pagerank, the importance of link popularity has increased significantly. These scripts allow you to get vital information on your site, and compare it with your competition.
Not only are these tools useful for domainers in picking up good quality PPC domains, but there also useful to webmasters looking to pickup a bit of extra traffic from mistypes.