Free SEO tools for websites

Mainly use the Google stuff
Google Analytics - great for site analysis (landing pages, exits)
Google Webmaster Tools - shows you how your site looks to google. keeps them happy.
Google Adword keyword tool - great for seeing which keywords are actually searched on!

Also use the SEOBook Rank Checker plugin for Firefox. Very handy to keep track of page positions.

SEO serp workbench is also great tool for seo persons. It gives the appropriate result for url for specific keyword.

There are literally hundreds of tools you can use to improve the SEO of your site. Here are some of my favourite ones:

(All are free or have free trials)

Link Assistant
SENuke X
Market Samurai
SEO Spyglass

use woo rank and it really a good website and free of cost.

I see this is getting replaced soon with the Keyword Planner tool, but apparently this will only be accessible for adword account holders, unlike the current keyword tool which is publicly available.

you can use raven tools for a month for free. it has limitations in the amount you can use in that first month, but its all there.

it can crawl the site and look for duplicates, missing tags, meta issues etc

you can also manage back-links with it and investigate the search terms that you want to target, and see where competitors are getting their backlinks from.

Woorank is great, though it's not totally free but it has 7 day free trial. The downside is that, within that trial duration, the services were limited and the results were very inconsistent. thumbsdown

Well Said sjr4x4!! It has been replaced with keyword planner tool and now troubling many SEO Experts. People are still struggling to understand the process of keyword planner tool (Me Too )

So, i have switched on Market Samurai some days ago. It is working fine and easy to understandable keyword research tool for me.

Spyglass is the free seo tools.

Webmaster tools / Google Adwords Keyword Tools
Xenu Link Sleuth
Screaming Frog SEO Spider