Make floating divs the same height

You could try instead of using float, use display: table-cell. You might find some older browsers don't understand this rule however. See below:

#wrapper {
    display: table; // See FelipeAls comment below
    width: 300px;
}

#left {
    display: table-cell;
    width: 50px;
    background: blue;
}

#right {
    display: table-cell;
    width: 250px;
    background: red;
}

Antony answer works ok, but you need all the divs to have the same parent and to have a wrapper, I have a solution that use javascript but works with any kind of element, they just need to have the same selector.

  function setEqualHeight(selector, triggerContinusly) {

    var elements = $(selector)
    elements.css("height", "auto")
    var max = Number.NEGATIVE_INFINITY;

    $.each(elements, function(index, item) {
        if ($(item).height() > max) {
            max = $(item).height()
        }
    })

    $(selector).css("height", max + "px")

    if (!!triggerContinusly) {
        $(document).on("input", selector, function() {
            setEqualHeight(selector, false)
        })

       $(window).resize(function() {
            setEqualHeight(selector, false)
       })
    }


}

    setEqualHeight(".sameh", true) 

http://jsfiddle.net/83WbS/2/