My favourite SEO Blogs

Search Engine Land - search news by Danny Sullivan and his team of SEO experts
SEOMoz Blog with Rand Fishkin
Aaron Wall's SEO Blog
Matt Cutts' Blog (for Google information)
Jim Boykin's Blog Matt McGee's
Small Business SEM
Problogger.net (for those with blogs)
Occam's Razor - web analytics blog by Avinash Kaushik
Official Google Webmaster Central Blog

Fun Tools for Site Assessment

Link validators: dead-links.com, elsop.com
Slow page load checker: websiteoptimization.com, netmechanic.com
Link popularity comparison: marketleap.com
Keyword density tools: live-keyword-analysis.com
Accessibility check: cynthiasays.com, w3.org
(for more links to accessibility evaluation tools)

Backlink Checker: backlinkwatch.com
Social Search Tag checker: socialmeter

More SEO Tools We Recommend
Wordtracker Keyword Tool for researching keyword search popularity. Wordtracker also offers a limited free keyword research tool. (This is an affiliate link so if you purchase access through this link, Wordtracker will show us a little love, too!)

Keyword Discovery - this fee-based keyword research tool has garnered many positive reviews - free trial available
Yahoo! Search Marketing's Keyword Selector Tool NOTE: this tool is no longer supported by Yahoo! and the link may not work
Google AdWords's Keyword Tool (can get some keyword ideas & relative search popularity numbers here)
Google AdWords's Keyword Selection Sandbox (can get some relative popularity values here)
SEOMoz Page Strength Tool (or see their other SEO Tools) > A truly fabulous spider emulator at seo-browser.com
MSN's set of SEO tools includes finding similar keywords, clustering keywords, and several others.
Rex Swain's HTTP Viewer is a handy way to check if your 301 redirects are sending the right server message.
TouchGraph Google Browser a cool way to visualize your website neighborhood

Socializer - links to social bookmarking sites