In general Google & other search engines are the primary self-marketers of websites. So, checking your website’s credibility at search engines is a good way of improving it.
Here are some major web-based (excluding softwares and browser extensions) pagerank check & seo tools websites which have resources like backlink analyzers, search engine position finders, and more.
Info: This list is not the "all SEO tools" list, but they are the ones which provide you many tools at the same place with an easy-to-use interface.
If you find these tools handy, bookmark the post at del.icio.us.
They have almost every seo tool you may need including keywork density checker and link popularity.
Besides the pagerank check and other webmaster tools, they have some advanced tools like mx / ns lookup, HTML Encryptor & more.
Although Khrido is not the most popular one, it is very user-friendly.
They have a social bookmark link checker, visual pagerank checker tool and a free seo report tool which are nice besides the standard seo tools.
SEOmoz provides advanced website analysis tools like finding how well your website is targeted for specific keywords or countries. Their pagerank checker & similar tools can be found here.
They are providing some tools for analyzing AdSense returns or comparing Google & Yahoo search results as a chart.
Besides the standard SEO tools, they have an unique keyword analysis tool & a Firefox pagerank checker extension that can be handy.
A great Google tool that gives you an idea about the popularity of keywords, and websites.
If you know of a great one please share at comments.