Here’s some great SEO and webmaster tools, all web based, all now linked from one handy spot.
Check your link popularity
Estimate the difficulty of achieving top ranking for a particular term
Find the age of a website (useful for google)
Simulate your site in Google (useful if not yet listed)
Find out your page size
View your server and browser headers
Code to text ratio calculator Aim for 50%
Excellent Keyword density checker
Google Page Rank Checker – great for mac users or if you don’t have google toolbar
Find your rank in Google
Find your rank in Yahoo