A seemingly simple question asked a while back at MezzoBlue, one that I struggled with myself for a long time before giving up and using tables:
Is it possible to use floats to position a fixed-width sidebar on the right of a page, with a liquid content area, if the content comes before the sidebar in the markup?
Floating, and not absolute positioning is necessary for the sake of a clearing footer.
1) no changing the order of the code (although adding new divs would be fine), and 2) no using absolute positioning unless you can somehow make it work with the footer.
Ryan Brill came up with an ingenious solution using negative margins that seems to work in almost all browsers. I have re-done the templates on this site with a variant of that method posted by Janos Horvath, which allows a background color to extend the full height of the page regardless of whether the content or the sidebar is longer than the other.
Finally! Table-free! (Hat tip: Richard Eriksson)