There's a common practice of using negative values for hiding. To hide an element while maintaining layout one might use something like:

.popup {
    position: absolute;
    top: -1000px;
    left: -1000px;
}

Another known trick is negative text-indent for hiding text while keeping an image (plus, this might hurt your ranking), e.g:

.logo {
    text-indent: -9999px;
    background-image: url("bla bla");
    overflow: hidden;
    width: 200px;
    height:60px;
}

The problem ? The above yields an ugly and large horizontal scroll bar when the document's direction is rtl.

To make things worst, some services (like ShareThis used in the blog), set those in scripts, which means it can't be overridden using css. I hide the scrollbar in rtl but the width is still there, scrolling sideways in the Hebrew version will reveal the fact.

There are solutions to those issues. For .popup above, just specifying the negative top and removing left from css will have same effect.

For .logo above I sometimes prefer:

.logo {
    background-image: url("bla bla");
    line-height:800%;
    overflow: hidden;
    width: 200px;
    height:60px;
}

This will keep the text below the height, and since overflow is hidden it won't be visible, while not breaking RTL documents.