Allan Jardine a web-developer and designer has developed some really helpful tools that are available for free on his site Spray Media.  I haven’t had a chance to explore all of the applications yet but I did go through the main design ones which include the Grid, Ruler, Cross-Hair and Unit.  As a designer these tools are helpful in many ways and are quick to access.  Simple drag the application name into your bookmark folder and each time you visit a site and are curious about the layout or just need to measure the size of something go to your bookmarks tab and the application will open up on that site.