Link Reputation / Link Popularity:

• Yahoo! Site Explorer
• Yahoo! Linkdomain: -internal links - forums - forum
• Yahoo! Special Linkdomain: (.gov .mil .edu .ac.uk)
• Yahoo! Link: - internal links
• MSN Linkdomain: -internal links - forums - forum
• MSN Link: -internal links
• TouchGraph For some reason Touchgraph kills the IE Beta on my computer (but this worked fine on older versions of IE) - provides a visual representation of linkage neighborhood using the Google TouchGraph.
• Link Harvester - provides an XLS sheet of links pointing at domain using Link Harvester.
• WeBuildPages - links to all Jim Boykin's cool tools.

Domain Age:

• Archive.org Domain - shows the history of a domain using Archive.org.
• Archive.org this Page - shows the history of a page using Archive.org.
• WhoIs Source - shows the WhoIs Source code for a site using WhoIs.sc.
• WhoIs Source History (Subscription Required) - shows the WhoIs history for a site via Whois.sc.

On Page Content Analysis:

• SEO Book Keyword Density Analysis
• Copyscape

Competitive Analysis:

• Alexa - shows traffic trends for a site from Alexa.com.
• Del.icio.us History - shows the history of the citation of a URL from Del.icio.us.
• Digital Point Keyword Ranking Tool - requires an account
• Myriad Search - shows the top ranking sites in the major search engines using Myriad Search.
• Google Trends - Historical trends for search queries.
• SEO Book Keyword Research Tool - shows keyword suggestions using the SEO Book keyword research tool.
• Google Keyword Research Tool - Shows Google's prices and volumes for keywords.
• Digital Point Keyword Suggestion Tool - keyword suggestion tool compares Yahoo! and Wordtracker side by side.
• Digital Point Keyword Ranking Tool - tracks your rankings in Google, Yahoo! and MSN.

Blogs Link Search:

• Technorati: Links to Page - shows blog links pointing at a page using Technorati.
• Technorati: Links to Domain - shows blog links pointing at a site using Technorati.

Official Search Engine Blogs:

• Ask Blog - The official blog about Ask.
• Google Blog - The official blog about Google.
• Google AdSense Blog - Blog about Google's contextual ad network.
• Google AdWords Blog - Blog about Google's advertising programs.
• MSN Search Blog -Official blog about MSN search
• Yahoo! Search Blog - Official blog about Yahoo! Search.

SEO & Search Related Blogs:

• David Naylor - The spamblog.
• Gaping Void - Hugh Macleod's cartoon blog on ad agencies and the advertising world.
• Google Blogoscoped - Philipp Lessen's blog on Google, search and the web.
• Gray Hat News - Funny blog about search.
• JenSense - Jen Slegg's blog about contextual ads.
• Jeremy Zawodny - Jeremy is a well known blogger and Yahoo! employee.
• Jim Boykin - Jim Boykin's blog about SEO.
• Link Building Blog - Blog by Patrick Gavin and Andy Hagans.
• Matt Cutts - Blog by a well known Google engineer.
• Oilman - Todd Friesen's blog about SEO.
• Performancing - Community blog about blogging (not really about SEO).
• SEO Black Hat - Blog about search spamming, etc.
• SEO Book - Aaron Wall's blog.
• SEO Buzz Box - Aaron Pratt's blog about SEO and the web.
• SEO Bytes - SEO forum aggregator.
• SEO Moz - Rand Fishkin's blog.
• SER - Barry Schwartz's blog on search.
• SEW - Danny Sullivan's Search Engine Watch blog.
• Searchblog - John Battelle's blog.
• Search Engine Blog - Peter Da Vanzo's blog.
• Seth Godin - Seth Godin's blog on marketing.
• Shoemoney - Jeremy Shoemaker's blog about marketing, programming and the web.
• Stuntdubl - Todd Malicoat's blog.
• Threadwatch - the Threadwatch community blog.
• TopRank Blog - Lee Odden's blog on public relations, blogging and SEO.
• Traffick - Andrew Goodman's blog primarily focused on paid search.
• Valley Wag - Unofficial gossip rag for Silicon Valley.
• V7N - John Scott's marketing blog and forum.
• Web Guerrilla - Greg Boser's blog on search and internet marketing.
• Wolf Howl - Graywolf's blog on SEO, marketing and blogging.