the world of web developers there are two standard ways of laying out a web page. The classical way is to use tables and structure your web page like you would an Excel Spreadsheet or a Word document. The second is to use Cascading Style Sheets that let you create elements on the page and position them according to your needs. For a large variety of reasons CSS based layouts are the way to go: they use less code, they are more customizable, they support various effects that aren't possible with tables, and they run faster in modern web browsers. The problem? They are darn hard to code!
Enter the CSS Grid Builder from the good folks at Yahoo!. Yahoo! has spent thousands of hours crafting web pages and testing them across all the possible OS and browser combinations (yes, even Opera). The end result of all this testing was the public release of the Yahoo! User Interface CSS and JavaScript libraries. The CSS Grid Builder is a simple web-based interface for quickly creating any number of layouts that rely solely on YUI's CSS files. This gives you the advantage of easily and visually laying out designs without using tables, and they will work the same way in every popular browser
http://developer.yahoo.com/yui/grids/builder/