SEO Backlink Tools

Backlink Watch
Domain Backlink Checker
Bad Neighborhood Tool
Broken Link Checker
Backlink Builder
SEOpro
Link Extractor
Domain-Pop
A list of more backlink tools.
Another list of more backlink tools.
Backlink Checking Tips
Majestic SEO Backlink Tool

SEO Keyword Tools

Google Sets
Adwords Keyword Tool
Microsoft Search Funnel
Entity Association Graph
Keyword Group Detection
Keyword Map
SEO Digger
Keyword Discovery
Keyword Density
Keyword Playground
Keyword Density Tool
SEO Book Keyword Suggestion
Keyword Density Check
Traffic Marks
SearchCloud 

SEO Analytic Tools

Compete.com
Quantcast.com
Google Webmaster Tools
Omniture.com
AdGooroo.com
LinkVendor.com
SiteYogi
Alexa.com
WebCEO.com
SiteRake
URLmetrix
Google Analytics
QuarkBase.com

SEO Engine Tools

SEO Browser
Spam Detector
Spider Simulator
Website Speed Test
Spider Simulator
Build Reciprocal Links
Spider Test

SEO Site Grader Tools

SEOmoz Trifecta Tool
http://www.websitegrader.com
WebSiteGrader.com
Webmaster Grade
Website Analyzer
SEO Scorecard
Cutop.com
Popuri.us
AideRSS.com

SEO SERP Tools

Shoemoney SERP Tool
Thumbshot Ranking
DigitalPoint SERP Tool
Google Dance
SearchBliss Google Dance
Samreen SERP Tool
SEOcase.net
Exact Factor.com
SheerSEO

SEO Firefox Tools

Search Status
SEO Quake
Rank Quest
SEOpen
SEO Tools
Kgen
SEObook for Firefox
Using Stylish to Show NoFollow
View Source Chart
JSview 2.0.5
HighLightAll
HTML Validator

Other SEO Tools

Page Rank Checker
Social Meter
Copy Scape
Duplicate Content Checker
Domain Stats Tool
Meta Tag Generator
DomainTools.com
HTML Encrypt Tool
Similar Page Checker
XinuReturns
Robots.txt File Generator
Robots.txt Analyzer
Cloak Checker
Metatag Extractor
Yahoo! Site Explorer
Class C Checker
Google Trends
Co.mments.com
Check-SEO
Niche Watch
List of SEO Friendly Directories
Mod-Rewrite Generator
Hosting History
Page Size Lookup
Banned Domain Checker
Domain Hacks Tool
DoFollow Blogs Search Engine
Vendor Discovery Tool 2.0
SEOmeter.com
TrackEngine.com
BuiltWith.com