One that doesn\'t require the following:
There is no such thing as "simple" when you talk about CSS.
But there is a simple solution that is cross browser, degrades gracefully and is fully HTML and CSS compliant: use a table with three columns.
Reasoning: DIVs are not meant to have the same dynamic height. Therefore, CSS has no support for this. If you need a block element which makes sure that its children have the same height, then that's what tables are for.
[EDIT] Sorry to offend all you CSS fanatics out there but, frankly, when something is not designed to do something and you abuse it, and it doesn't work, please stop complaining, ok? A DIV is not a TABLE and can't be used as one without relying on hacks.
[EDIT2] As was said already in various places, the reason not to use tables for layout was that, in early times, tables were the only design element and that lead to HTML which had dozens of nested tables. That's bad. But that doesn't mean you must not use a single table to put everything in place and then rely on CSS to make the stuff inside look right.
A brain is like a parachute: It's nice to have but only useful when it's open.