Fluid / liquid or responsive layout? There should be no question about this, as nowadays the screen resolutions vary from 800 (yes, still in use) to 2400 pixels width, imagine how bad a responsive 900 px layout looks in a 28 inch browser window? A liquid (or elastic as Roger Johansson mentioned ) layout will always adjust itself on the browser window, only IE 6 makes problems as always: IE Doubled Float-Margin Bug.
Glish describes a four layout column technique that can actually be used to provide as many columns on a page as you like.
Max design explains one method of achieving a successful liquid layout the easy way.
Creating Liquid Layouts with Negative Margins by A list apart is providing some useful tips on creating a fluid layout with negative margins, which allow pushing the content area away from the sides of the browser and leaving room for the sidebar.
Liquid Designs ( created by Christian Montoya ) has a nice showcase of the best fluid designs including mine : 1200, 1024 and 800pixels browser size. Screenshots are taken with Firefox, but IE (all orphans) shows it the same way.
Four layout column technique