Free SEO Tools

Domain Statistics
 
Domain Age Tool - Check the age of any domain
Domain Stats Tool - Provides domain age, Alexa Rank, Google Page Rank, and number of incomming links
Website’s Page Rank
 
Google Page Rank Checker - Check the Google Page Rank of any site
Alexa Rank Checker - Tool to check your Alexa Ranking

Backlinks
 
Backlink Summary - Discover how many links are coming in to your site
Backlink Anchor Text Analyzer - Find out what words people are using to link to your site
Backlink Checker - Find out how many sites are linking to yours

Free Keyword Research Tools
 
Google AdWords Keyword Tool - Research your keywords, and find how competitive they are on Google
Keyword Phrase List Generator - Helps you generate keyword phrases from the keywords you type in
MIVA Keyword Generator - Find out how popular your keywords are
Overture Keywords Suggestion Tool - Gives you similar keywords plus their costs in the Overture system
Search Engine Keyword Tracker & Keyword Ranking Tool - Utility to track historical performance of your keywords
Website Keyword Suggestions - Offers keyword suggestions based on the content of your website
Keyword Playground - Offers keyword suggestions based on monthly keyword performance
Free Keyword Analysis Tool
 
Keyword Density Checker - Check the keyword density for any web page
SEO Tools: - AdWords / Overture Keywords Generator, Keyword Search Count Scraper, Keyword Selection Tool
VirtualPromote Keyword Density Analyzer - Analyze the keyword density of any webpage
Current Keyword Searches (Search Engine Spies)
 
Ask Jeeves Take A Peek - Top searches of the week
Dogpile: Popular Searches - The most popular searches on Dogpile
Google Zeitgeist - The most popular searches on Google sorted by day, week, month, or year
Metacrawler MetaSpy - See what people are searching for on Metacrawler
Search.com Top 100 Searches - The top 100 search terms of the week
The Lycos 50 Daily Report - The top 50 most popular searches on Lycos each day
Yahoo! Buzz Index - Shows what people are searching for most on Yahoo!